Why the United States Is Functionally Impossible to Invade

Invasion. It is the shadowy threat that has loomed over the United States since its inception — the deep, fearsome prospect that has kept generation after generation of general and president awake at night. From its early days as a fragile republic, through the years it was torn asunder by civil war, and even now in its full form as a modern superpower, the country has always feared the prospect of an enemy march on Washington. The great American experiment, as it is so often called, has been running for nearly two hundred and fifty years — yet the notion that it could all be ground into dust if its people aren’t careful has never quite gone away.

In truth, however, a full-scale military invasion is something the United States has not had to seriously worry about for generations. The modern American homeland is arguably the least invadable place human civilization has ever created, and by a long margin. From a geography that is simply impossible to match, to a population ready to resist at a moment’s notice, to a carefully tailored list of adversaries and a deliberately sneaky approach to grand strategy, the country has assembled the best home-field advantage in history.

This is not a story about American invincibility in the abstract. It is a story about layers — interlocking rings of defense that an attacker would have to peel back one by one, each more punishing than the last, before reaching anything that could be called a victory. The thesis is straightforward, and the rest follows from it: a full-scale military invasion of the United States is not merely difficult — it is, for any nation or coalition on Earth, functionally impossible.

Key Takeaways

The United States should be understood as a castle defended by interlocking rings — a global network of forward bases and island territories, the buffer of two oceans, dominance in space-based surveillance, formidable interior terrain, an armed population, and the active-duty military and its reserves.
Roughly 160,000 American troops are stationed outside the homeland in dozens of nations, alongside island territories like Guam, Hawai’i, and Alaska and nine carrier strike groups — all of which an invader must defeat before even reaching the mainland.
Geography does the heavy lifting: any seaborne attack must cross hundreds to thousands of miles of open ocean against vulnerable supply lines, while landed forces confront the Pacific Coast Ranges, the Sierra Nevada or Cascades, the Rockies, the Appalachians, deserts, bayous, and fortified cities.
The hard arithmetic is decisive: even under absurdly generous assumptions, China — the world’s runner-up in amphibious capacity — could land only about 48,700 troops, 1,500 heavy vehicles, and roughly 100 aircraft, nowhere near enough to subdue the homeland.
Because invasion is impossible, adversaries turn to other tools — nuclear deterrence, economic warfare, cyberattacks, sabotage, terrorism, and luring the United States into costly wars abroad.

Every Country Is a Castle

Every country is a castle. It is as true today as it was in ancient or medieval times, even if borders, populations, and technology have all shifted with the centuries. Above all else, a nation must protect its sovereignty — whether by military defense, by alliance, by integration, or by whatever other means it chooses. A nation’s specific methods are its own, but the mission is constant: maintain independence, protect it, and after that, leave the rest to a country’s leaders and people.

And if every country is a castle, then the castle the United States has built cannot reasonably be compared to any other.

The most useful way to think about American defense is as a series of interlocking rings. A historical castle is defended first on the surrounding battlefields, then while retreating toward a highly defensible point. It has exterior ditches or moats, then a vast outer wall, then inner walls, and a whole range of traps, chokepoints, and defensible areas — all before an adversary can finally break through to the keep and declare victory. American resistance to invasion works the same way, only on a continental and even global scale.

And it begins not at the coastline but with American presence all around the world.

The Outer Ring: A World of Mini-Americas

Declare war on the United States with the intention of one day invading the mainland, and long before you get close, you must deal with an enormous number of what can be called “mini-Americas” — the country’s foreign bases, its island territories, and the floating cities that are its carrier strike groups.

In Japan alone, the United States stations more than fifty thousand troops, including an aircraft carrier, many large naval vessels, several advanced air combat units, and more. They are backed by twenty-four thousand additional troops in South Korea, including an entire field army’s worth of soldiers, major military intelligence capabilities, and enough lethal aircraft to ruin far more than your day. Germany hosts thirty-five thousand; Italy and the United Kingdom each have over ten thousand; and Turkey, Spain, Belgium, and Bahrain each host a thousand or more.

All told, the United States stations troops in dozens of nations, including several where those forces are deployed into active combat. Start a war with America, and every one of those troops — some 160,000 in total, joined by all manner of advanced warfighting equipment — will immediately set about becoming a problem. And they can.

Then there are American defenses on its own territory and in international waters, far beyond the contiguous forty-eight states. Consider Guam, an island and territory closer to China, Australia, or the Philippines than it is to Los Angeles. The Navy, Air Force, and Marines each maintain a formidable presence there, including an anticipated five thousand Marines in the coming years, several nuclear submarines, and a rotating force of strategic bombers. The Hawaiian archipelago is even more formidable, with F-22 Raptors, tens of thousands of garrisoned soldiers and Marines, and the entire Pacific Fleet.

Hawai’i may be a tropical paradise, but it is not a place to pick a fight. Alaska, in the high north, presents a similar situation.

Making matters worse, America also has its own floating islands — hundreds of them, including nearly a dozen otherwise known as aircraft carriers. A modern Nimitz-class carrier can hold five thousand crew members, dozens of guided missiles, and anywhere from sixty-four to one hundred and thirty warplanes, depending on what the Navy has chosen to deploy ship to ship. The more advanced Gerald R.

Ford-class can carry up to ninety aircraft and even more missiles, while fighting better and sustaining operations longer than its predecessor. Nor do these beasts travel alone; each sails alongside guided-missile cruisers, advanced destroyers, and, quite likely, nuclear submarines.

What all of those assets mean — the foreign bases, the built-up territories, the carrier strike groups — is that before an enemy can touch the mainland, it must engage in a maddening version of island-hopping. No matter who attempts an invasion, they must first deal with nearby bases on land, sometimes home to thousands of soldiers, then get past every layer of defense the United States can erect in the open ocean, on or around island territories, and out of foreign bases elsewhere capable of projecting force across great distances. Clear that entire sequence, and yes, you might reach the mainland — but accomplishing it is a brutally difficult task for any military on Earth. And the problem is made far worse by two factors not yet counted: the immense natural advantage of the world’s oceans, and America’s all-seeing dominance in space.

The Moat: Two Oceans and an Unblinking Eye

Any nation that does not plan to attack through Canada or Mexico faces a massive problem. Apart from a single exception at the Bering Strait — between Alaska and the far Russian east, and unsuitable for large-scale military operations anyway — any seaborne attack against the United States must cross hundreds to thousands of miles of open ocean. Whether from the Atlantic or the Pacific, the surrounding waters are vast, and the difficulties go well beyond the punishing island-hopping already described.

Watch on WarFronts

Watch the full video analysis on the WarFronts YouTube channel, presented by Simon Whistler.

Even under ideal conditions, a nation can only reach America by sea if it can sustain a logistical operation across a major world ocean. During the crossing, and once forces hypothetically established a presence on the mainland, those invaders would have to be supplied and protected across incredibly long lines. This is not an operation on the scale of the D-Day landings, which exploited a relatively narrow English Channel and a major military power’s coastline nearby.

It is the construction of supply bridges across an exponentially greater divide — lines that are both logistically punishing to sustain and acutely vulnerable to constant hit-and-run attacks. Those attacks are far easier to launch than to defend against, especially for an adversary, like the United States, that fields nuclear-powered submarines. An invading force can also transport only as many troops and as much hardware as it can carry across that distance — a constraint with consequences explored further on.

And if all of that were not difficult enough, any invader would have to manage that ocean-crossing, island-hopping operation with the American military fully aware of where they are, what they are doing, and where they are going next. The country’s surveillance capabilities are second to none, most notably in space, where a range of classified satellite intelligence programs can capture imagery and signals data — most likely in real time — at numerous points of interest worldwide, all at once. Other reconnaissance assets include highly advanced spy drones, extreme high-altitude aircraft, and shadowy organizations like the Central Intelligence Agency, more than capable of leveraging human assets across the globe. With those abilities, the United States can target and strike an adversary’s invasion infrastructure with exceptional precision — to a degree where any sober military strategist would conclude that the effort simply isn’t worth it.

Home-Court Advantage: The Walls of Terrain

Suppose an attacker somehow clears the sea-based and international rings and lands a force on American soil — on the East Coast, the West Coast, near the Canadian border, or near the Mexican border. Now what? The answer depends on the direction of attack, but no matter how you slice it, there is no shortage of natural barriers forming this next inner ring of defense.

Come from the West Coast, and wherever you land, you must breach multiple mountain ranges to reach the heartland. First are the Pacific Coast Ranges, running from Alaska to central Mexico and set so close to the sea in places that you can descend straight from the high coastal hills into the Pacific. Many sections are extremely hard to traverse, with only a handful of passes where tanks or heavy artillery could safely move.

Push through, and depending on where you landed, you reach Level Two: the Sierra Nevada toward the south or the Cascades to the north — choppier, higher in altitude, and deeper than the coastal ranges in most places. Beyond those lies the toughest challenge of all, the Rockies, North America’s largest mountain system, with an abundance of peaks above five thousand feet, or 1,500 meters.

Attack from the East Coast and the situation is marginally better, but not much. Strike in the north, near New England, and you soon run into the White Mountains, then the Green Mountains and Berkshires, then the Taconics and the Adirondacks. Strike farther south, in the mid-Atlantic or the Carolinas, and you gain more distance inland — only to meet the heart of the Appalachians, lower in elevation than the Rockies but thickly forested and very difficult to cross.

The Appalachians are such a headache that even today transport infrastructure in that region is sparse. Attack from Alaska, working down through Canada, and the path is no easier; you simply follow the Rockies for most of it, after first getting through the bitterly cold Alaskan ranges, practically overflowing with bears and other large wildlife. Even an attempt to strike north from the Gulf states runs through the Ouachita Mountains, a low-lying but hilly range with enough natural obstacles to become a major problem.

The specifics of mountain warfare are a subject in their own right — WarFronts has examined them at length as part of its Art of War series. But the short version is this: no matter the range, mountains are a devilishly difficult enemy. They are hard, often impossible, to move large vehicles through. They are replete with chokepoints where a defender can stage attacks.

They are ideal terrain for asymmetric defenders who know the land to wage a guerrilla campaign. They eat into the time an invasion requires, giving the defender room to react and consolidate. And particularly high, cold ranges add their own dangers — altitude sickness, frostbite, avalanches — while forcing attackers to spend yet more time acclimatizing.

The Barrier Lands: Deserts, Bayous, and Cities

Even an invader who tried to go around the mountains would have few good options. The American Southwest — California, Arizona, New Mexico, Nevada — is largely scorching desert, where conditions can be brutal, infrastructure is scarce, and shelter from the elements barely exists. The southeastern states of Louisiana and Florida are protected by mucky, hard-to-penetrate bayous filled with alligators and parasites; any force that gets through the bayous lands in mosquito country that defies description. Come down from the north, and an attacker confronts thick forests in a land pockmarked by lakes, making for a slow advance along shaped, predictable pathways that are easy to defend.

And all along the coasts sit the cities — sprawling Los Angeles and Houston, tightly packed New York, San Francisco, Boston, Seattle, and more. To reach the main road routes into the heartland, you have to go through urban areas, and to attempt that is to take on the incredibly difficult task of urban warfare. Each of these barrier lands compounds the last. The United States is not merely hard to attack; it is easy to defend, because its terrain channels any advance into precisely the kind of slow, exposed, predictable movement that favors a prepared defender.

The Keep: The Heartland Fortress

From there, things become even more frustrating for an attacker. Cross America’s many barrier lands, and you reach the heartland — broadly, the Midwest — an innermost ring that could hardly be a better place to endure a siege if it had been designed for the purpose. This is the breadbasket of the United States, with immense agricultural potential and the capacity to sustain the country’s population for a very long time, especially under war conditions.

That same heartland is the seat of America’s industrial base, especially its military manufacturing. M1 Abrams tanks are made exclusively in the heartland state of Ohio. Artillery and shells come from Pennsylvania, Arkansas, and Iowa. States like Missouri and Kansas are hubs for aerospace manufacturing and innovation.

The region hosts hundreds upon hundreds of small-arms manufacturers, and it is where America’s most potent weapons — its land-based nuclear arsenal — are kept safe and ready. Crucially, Americans can reach all of it rapidly, using the interstate road system, the same network that lets the military pivot its defense quickly in a crisis.

The interstates could not, admittedly, manage something like a twelve-hour evacuation of the coasts. But that scenario never arises, because of all the surveillance capabilities already described. Americans would not have hours of warning; they would have weeks. And when they arrived in the interior, they would find no shortage of land waiting for them — defensible, productive, and far from the sea.

The Last Wall: 120 Million People and 1.3 Million Troops

Finally, there are the people. Resistance to invasion and the principle of armed self-protection have been central to America’s ethos and self-concept for centuries, and even the civilian population has the weaponry to match. Per capita, the United States has more firearms than any other nation on Earth — enough guns to put a weapon in the hands of every adult, child, and infant, with an estimated sixty-five million or more left over.

Not everyone is qualified or able to use those firearms, but according to publicly available figures from the Central Intelligence Agency, some 120 million American men and women between the ages of sixteen and forty-nine are believed fit for military service. Imagine those 120 million people forced from their homes and pushed back toward the heartland, with everyone and everything they have ever known under threat. They will be furious.

Mock Americans for their McDonald’s all you like — but do not underestimate their willingness to take that McDonald’s back by force.

And remember, there is still the formal military to contend with: roughly 1.3 million active-duty service members and eight hundred thousand reservists. At their disposal would be the vast majority of America’s global military hardware — over eight thousand main battle tanks, hundreds of missile defense systems, several thousand fighter and bomber aircraft — plus an estimated fifteen million veterans, many of whom could re-enter service with minimal retraining or take on the task of preparing fellow citizens to defend the homeland. Defeat all of that, and you have finally made it past the innermost ring of American defense.

That assumes, of course, that you have somehow defused the nuclear weapons on the way in, without America’s leadership deciding to press the proverbial big red button and wipe out an invading force outright. Fail to break past every single ring described here, and the invasion simply fails.

To Conquer an Eagle: The Skeptic’s Test

Those are the reasons the United States is very, very hard to invade. But a healthy skepticism is in order, because nothing above quite reaches the word impossible. After all, if an adversary could somehow get through every challenge, could it not complete a successful invasion? Strictly speaking, yes.

But if a successful invasion under those terms sounds like something any nation could actually achieve, then one of two critical errors is being made. Either the resources required are being wildly underestimated, or the capabilities of adversarial nations to muster those resources are being massively overestimated.

This is where the analysis pivots from the staggering defensive attributes of the United States to the equally staggering requirements an attacker would have to meet — beginning with alliances. If the Atlantic and Pacific are America’s guardians to the east and west, then Mexico and Canada are its guardians to the north and south. Diplomatic disagreements notwithstanding, the United States, Canada, and Mexico are inextricably linked through trade, military and economic collaboration, culture, and far more.

It is flat-out inconceivable that any adversary could win Canadian or Mexican consent even to serve as the launch point for a small cross-border raid against an American town, let alone to mass troops for a full-scale invasion. Both neighbors benefit enormously from America’s nuclear umbrella and its ability to guarantee regional security, and the sheer military disparity between the United States and its neighbors gives both very good reason to ensure Washington never perceives a major threat emanating from their soil. The idea of North America as an impenetrable fortress is good for every nation on the continent.

With Mexican or Canadian help off the table, any land invasion would require building up forces either in remote areas of Canada or Alaska in the north, or somewhere in Central or South America, and then beginning a long march toward American soil. But as already established, the Americans would see you coming — and survival becomes a grim proposition once they do. With those staging areas well within range of American missiles, aircraft, and offshore naval assets, and with an invading force only easier to target as it draws closer, the full might of the American military could respond to an approaching land threat before it ever reached the mainland.

Whether Canada or Mexico would be enthusiastic about burned-out tank columns and vast expenditures of munitions on their territory is an open question. But from Washington’s perspective, being the continent’s heaviest hitter by an order of magnitude carries a fringe benefit: a bit more leeway to act first and apologize to the neighbors afterward.

The Arithmetic of the Sea: Why the Numbers Don’t Add Up

If invasion through Canada and Mexico is a non-starter, the next-best option is a seaborne assault — again, assuming all those forward-stationed forces, island and territorial holdings, and roving carrier strike groups have somehow been neutralized. To illustrate the problem point by point, grant an absurdly favorable assumption: that an invading force has, by some stroke of luck, massed in Cuba intending to strike at the Everglades in southern Florida. That staging alone would be a logistical nightmare, but set that aside. In that case, a sea invasion would need to cover only about 180 miles of the Atlantic — roughly 300 kilometers, merely double the distance of the D-Day landings.

The next constraint is amphibious assault craft, and here no single nation on Earth has the capacity to ferry enough troops to American soil at once. The United States, unsurprisingly, fields the highest number of amphibious assault ships in the world. But consider the runner-up, China, whose amphibious capability is what allows it to threaten a full-scale invasion of Taiwan.

China has four copies of its largest amphibious assault ship, the Type 075, each able to carry sixty armored fighting vehicles, eight hundred troops, and twenty-eight helicopters. Its next-largest, the Type 071, comes in eight copies, again carrying sixty armored vehicles and eight hundred troops, plus four helicopters apiece. A much larger ship, the Type 076, is expected to carry over a thousand Chinese Marines per ship and to launch and recover unmanned drones — but only one has been launched so far, and it is not expected to be battle-ready until 2027.

China also has roughly seventy other large landing vessels, typically carrying about ten armored vehicles and 250 troops each, though a few carry more.

Now stack the assumptions in China’s favor. Bump the totals up by about twenty-five percent for the smaller vessels. Assume the largest ship is already operational. Assume China can commit one hundred percent of its amphibious craft to a single invasion. Assume it can muster enough smaller or civilian craft to add fifty percent more troop-landing capacity. And assume its soon-to-be-three aircraft carriers, plus several dozen destroyers and frigates, can keep the landing force completely safe on approach.

Even in that incredibly favorable, frankly unrealistic scenario, China could perhaps land about 48,700 troops, 1,500 heavy fighting vehicles, and roughly 300 helicopters on American soil — plus air support from its two functioning carriers and one undergoing sea trials, combining for maybe a hundred aircraft. That is a highly formidable force. It is also nowhere near what would be required to face down the American military on its own ground.

Even under perfect conditions, it would take a miracle for such a force to fight its way up the Florida peninsula with the full weight of the United States bearing down. Other modern adversaries — Russia, Iran, North Korea — could not come close to those numbers. And even if all of them worked together under the same perfect conditions granted to China, it would remain a losing effort.

Strip away those perfect conditions — discard the assumption that the United States would ignore a force massing in Cuba, or that anyone could simply walk through the Everglades — and the scenario becomes truly impossible. None of those nations could sustain an attack force of comparable size as it stretched across an ocean. None could stop American naval air power, let alone the country’s nuclear submarines, from picking off landing craft and support ships during a long Atlantic crossing — or, worse, a Pacific one.

At best, such forces could pull off a beach landing of a few thousand troops at highly remote locations, and only if the United States chose to let them have the landing zone. Contest those zones seriously with the resources America has on hand, and it would be over quickly.

No Way In By Air, and No Way to Hold the Ground

An air invasion fares no better. Strategic airlifters like Russia’s Il-76 or China’s Y-20 could only make such a flight if they fully intended to run out of fuel and crash, and only if they were willing to brave American airspace without escort — neither country fields fighter aircraft capable of guarding these planes across such long distances. Even if both nations sacrificed every strategic airlifter they had, and even if, miraculously, none of the undefended, unarmed aircraft were shot down, they could deposit perhaps twenty-two thousand unlucky paratroopers with no hope of backup or support. If a nation actually wanted its airlifters to survive and return to base, the math collapses further: Russia’s grand total of four airworthy Antonov An-124s — again, with zero protection — could drop a paltry 1,400 paratroopers to face a frankly embarrassing fate.

And all of this ignores the sheer number of troops that would be required on American soil simply to hold territory and put down resistance after a takeover. For reference, at the height of the Iraq War the United States stationed over 170,000 troops in the country and still could not prevent a long-term return to violence — and Iraq is barely two-thirds the size of the single state of Texas. The prospect of any world nation even being in a position to face that problem is so remote that the most honest advice is not to worry about it.

The logistical challenges of placing adversary troops on American soil are so prohibitive that even a fraction of American forces could handily defeat the largest invading army another nation could plausibly land. Russia could not do it. China could not do it.

Russia and China together could not do it. A hypothetically hostile Europe could not do it. India, Brazil, Iran, North Korea — it is simply not happening.

Undermine and Whittle Away: The Real Threats

It would be a mistake to conclude without attention to a broader reality. Just because the United States cannot be properly invaded does not mean it cannot be attacked. In fact, the impossibility of a military invasion is a double-edged sword. Yes, the country can rest assured it will not face invasion for at least several decades. But precisely because that avenue is closed, America’s adversaries have stronger incentives to develop alternative modes of attack.

Some nations — the Soviet Union of old, the Russia and China of today — pour themselves into nuclear proliferation. Russia and the United States currently deploy nearly the same number of nuclear weapons, with China on pace to match both within a decade or two as it rapidly expands its stockpile. Neither Russia nor China can threaten the United States with invasion, but both can threaten to do it incredible harm, recouping by another route the deterrent and negotiating power that comes with the ability to directly hurt an adversary.

Economic warfare is another potent tool, and one that the United States and its rivals appear set to keep wielding in the years ahead — from American-led sanctions against Russia after the invasion of Ukraine to trade wars between Beijing and Washington. Nations can inflict real economic damage on the United States, though usually not without suffering considerable damage themselves.

Other adversaries may reach for the newer weapons of hybrid warfare — cyberattacks and quiet infiltration of sensitive systems. Over the past year, the United States has learned more and more about the ways China has penetrated critical American infrastructure to seek out valuable intelligence, all without a single person setting foot on American soil. A nation might also turn to sabotage, much as American domestic extremists have increasingly targeted the power grid, water infrastructure, and more.

And then there is terrorism: a nation or non-state actor seeking to harm the United States might do so through a terror attack. One need only recall the events of September 11, 2001, to understand the sheer potency of that weapon against America.

Finally, nations that seek to diminish American military might can do it on their own terms — luring the United States into unfavorable engagements abroad rather than attacking it at home. Consider the success of guerrilla forces against American troops in Vietnam, in Afghanistan, and elsewhere over the past half-century. If an adversary can get American forces onto its own soil, then those forces — and American interests more broadly — become far easier to damage. Imagine what China could achieve by luring the United States into a war on its own terms over Taiwan and the South Pacific, compared with what it could ever accomplish by attacking America outright.

Why It Simply Won’t Happen

These alternatives matter not only because they are real mechanisms for harming the United States without invading it, but because they constitute the final reason an all-out invasion is so implausible for any nation to attempt. It is nearly impossible to get troops to American soil. It is nearly impossible to achieve military victory once there. And on top of both, an adversary does not need to bother.

If you can have the same reward by asking nicely as by walking the length of the Orient Express — with no improvement in the prize for taking the harder road — the choice makes itself. You ask nicely, take the reward, and decline to expend enormous effort on an endeavor that makes the outcome no sweeter.

So while there are many reasons a military invasion of the United States is functionally impossible, this is the reason it will not be tried. Any nation or non-state actor that wishes to harm the United States can spare itself a great deal of pain, tremendous losses, and considerable embarrassment by going about it some other way. There are, occasionally, leaders in this world who need reminding that invading another country is a poor idea — Vladimir Putin among them. But when the country in question is the United States, it is crystal-clear to anyone who would ever consider it: invasion simply is not worth the trouble.

Presented by

Simon Whistler

Simon Whistler is one of YouTube's most prolific educational creators. WarFronts is his deep dive into military history and conflict analysis.

Frequently Asked Questions

How should the American defense posture be understood as a whole?

As a series of interlocking rings, like a castle. The outermost ring is America’s global presence — forward bases, island territories, and carrier strike groups. Beyond that lie the oceans and space-based surveillance, then the interior terrain of mountains, deserts, bayous, forests, and cities, then the heartland redoubt, and finally the armed population and the formal military. An attacker would have to defeat each ring in sequence before reaching anything that could be called a victory.

How many American troops are stationed abroad, and why does that matter for an invasion?

Roughly 160,000 troops are stationed outside the homeland across dozens of nations — including more than fifty thousand in Japan, twenty-four thousand in South Korea, thirty-five thousand in Germany, over ten thousand each in Italy and the United Kingdom, and a thousand or more in Turkey, Spain, Belgium, and Bahrain. Combined with island territories like Guam, Hawai’i, and Alaska and nine carrier strike groups, these forces must be defeated before an enemy can even reach the mainland — a punishing form of island-hopping before any ocean crossing begins.

What does the amphibious math show about China, the world’s runner-up in sealift?

Even under wildly generous assumptions — counting the not-yet-ready Type 076 as operational, committing one hundred percent of China’s amphibious craft, adding fifty percent more capacity from smaller and civilian craft, and assuming its carriers and escorts keep the force safe — China could land only about 48,700 troops, 1,500 heavy fighting vehicles, and roughly 300 helicopters, with about a hundred aircraft in support. That is a formidable force but nowhere near enough to defeat the American military on its own soil, let alone hold territory against 120 million service-eligible adults and 1.3 million active-duty personnel.

Even if troops landed, why couldn’t an invader hold American territory?

Holding ground requires enormous occupying forces. At the height of the Iraq War the United States stationed over 170,000 troops in a country barely two-thirds the size of Texas and still could not prevent a long-term return to violence. The American homeland is vastly larger, far more populous, and far better defended, with a heartland that produces its own food, weapons, and nuclear arsenal, an interstate road network allowing rapid military redeployment, and an armed civilian population estimated at 120 million service-eligible adults.

If invasion is impossible, how do adversaries try to harm the United States instead?

Because military invasion is closed off, adversaries pursue nuclear deterrence — Russia at near-parity in deployed warheads, China expanding its stockpile rapidly — along with economic warfare, cyberattacks and infiltration of critical infrastructure, sabotage of the power grid and water systems, terrorism as demonstrated on September 11, 2001, and by luring the United States into costly engagements abroad, as guerrilla forces did in Vietnam and Afghanistan.