What Would Happen If The US Were Invaded
The United States has been invaded a few times in its history, but no invasion has ever been able to make headway, let alone occupy the lower 48 since the War of 1812. That has not stopped foreign countries from trying to plan an invasion of the U.S. Imperial Germany, Canada, the USSR, and the Axis powers all had war plans for such an event, but none of them ever materialized.
The mainland United States has two huge borders, but it is separated from Eurasia by two oceans and has numerous other deterrents in place that would pulverize any invasion attempts of the U.S. mainland. Any enemy would have to break through layer after layer of defenses and deterrents, most of them abroad, before having a chance at even setting foot on U.S. soil. Here is how the United States would react to a potential invasion and why ultimately, it would be very difficult for anything other than an extraterrestrial invasion to successfully attack and occupy the country through conventional means.
Nuclear deterrents
Because the United States' strategy for national defense is focused on deterrence and containment, any invasion of U.S. territory, whether on the mainland or otherwise, would face the major hurdle of a nuclear arsenal. Abdul Qadeer Khan, the father of Pakistan's nuclear program, wrote in Newsweek that no nuclear-armed country has ever been invaded or occupied. The United States certainly fits this bill and has employed the strategy since the Cold War, when it was known as the doctrine of mutually assured destruction.
The Arms Control Association notes that the United States possesses the second-biggest nuclear arsenal after Russia. According to War on the Rocks, this large arsenal serves the principle of mutually assured destruction if the U.S. encounters with potential rival hegemons such as Russia and China, a policy dating back to the Cold War under the Eisenhower and Kennedy administrations. Ideally, if Russia or another state launched a nuclear attack against the United States, the retaliation would annihilate the enemy many times over.
According to Dr. Matthew Kroenig's testimony before the Senate Armed Services Committee, these nuclear weapons allow the United States to strike back against any enemy anywhere in protection of any allies and of itself with the goal of crippling their ability to wage war. Thus, they serve as a deterrent against nuclear attacks on U.S. soil, but logically also against any ground invasions of U.S. territory. In this regard, Khan's words ring true. No one will ever attempt to invade the United States if the price is a barrage of nuclear missiles.
United States bases abroad
Just as the U.S. nuclear arsenal is officially used to deter attacks against the "entire free world" and the U.S. mainland, per Dr. Matthew Kroenig, the 800 or so U.S. bases around the world serve a similar purpose. According to Raphael Cohen (via War on the Rocks), United States bases abroad serve not only to project power, but to deter enemies from invading and occupying countries allied to the United States, such as South Korea, Poland, Turkey, the Baltic region, or the heart of NATO, western Europe.
When threats to security do arise, the argument goes that these bases allow U.S. soldiers to quickly deploy overseas and neutralize threats at their source (via the Brookings Institution). These bases are portrayed mostly as launching pads for offensive war. However, it follows that such a policy serves to keep conflict from ever reaching the U.S. mainland. The threat of United States forces quickly striking into enemy territory would be the principal deterrent here.
Some organizations, such as the Cato Institute, have proposed closing U.S military bases. The institute argues that the bases are not needed as a deterrent because the United States is already separated geographically from Eurasia by two oceans, making it difficult to invade. That is, that the U.S. is already plenty secure and does not need a forward deployment posture abroad to protect the mainland.
The Pacific Fleet
While the United States' defense strategy has focused on squelching threats abroad before they ever reach the mainland U.S., there is a line of defense to prevent invasions from the west, which in today's day in age would most likely originate in China or Russia. According to Yahoo! News, Japanese official Yasuhide Nakayama warned that both countries had developed plans to threaten not only Taiwan, but also Hawaii, the United States' first line of defense in case of a conventional military threat to the U.S. mainland. Russian and Chinese naval drills have raised fears of growing Chinese influence from Hawaii to Japan, a situation that would force the United States into a defense of the West Coast.
According to Sea Power Magazine, the primary duty of containing Chinese power falls to the U.S. Pacific Fleet. Although Taiwan is the first line of defense, the USNI notes that any conflict would come to threaten Guam and Hawaii too. In a naval contest, it is unclear who would come out on top. Much of that comes down to where the conflict is fought. According to National Defense Magazine, the United States' strategy would be to contain any Chinese attacks before they hit the Pacific and the U.S. mainland. The Defense Department has set up the Pacific Deterrence Initiative to prevent exactly this sort of scenario, but if it fails, the mainland would theoretically be at risk of invasion, if an enemy were logistically capable of mounting one.
NATO would likely have to disappear
NATO has seen much scrutiny in recent years, with some calling for the organization to be disbanded. President Donald Trump argued that the alliance was even obsolete, and he is not alone in that regard. Those favorable to NATO, however, argue that the alliance serves as a barrier to invasion against the East Coast since any invading enemy would have to move through western Europe first.
According to The World, NATO was founded to prevent Soviet dominance over war-torn western Europe following World War II. Logic dictated that a Soviet-dominated Europe would pose a threat to U.S. soil. According to the Atlantic, during the Cold War, the USSR did have plans for an invasion through the East Coast and Greenland, so those fears probably had some realistic background to them. Thus, if all NATO countries in Europe were formed into a defensive bloc strengthened with U.S. bases and backed with nuclear deterrence ("massive retaliation"), the USSR would never occupy western Europe or U.S soil.
As the map shows, NATO would need to be crushed first before a realistic invasion of the United States from Europe could occur. U.S. forces would need to be pushed out from Europe, and the only power capable of accomplishing this is Russia. War on the Rocks notes that the Baltic could serve as the flashpoint. NATO has taken steps to secure the small Baltic Republics from Russian interference through missile and conventional deterrents. Now, even if NATO was shattered, there would still be one more major obstacle to any invasion of America: Greenland.
Greenland
Greenland has long been recognized as a possible launching pad for invasion, since at least 1941. According to the US Coast Guard Aviation Association, German activities in Greenland led the United States to establish its military presence there. The Atlantic notes that the Soviets also planned to use the island and the Arctic in an invasion of the United States if it ever came to that. Thus, the United States maintains a military presence on Greenland to ensure that should NATO fall, there is still another defense line to fall back on. Recent interests in buying the island suggest that it is still crucial to national defense.
Today, Greenland has become the battleground between the U.S., Russia, and China over Arctic resources and strategic positions. The Wall Street Journal notes that Russia has aimed to outmuscle the United States in the Arctic by building up its military capacity to threaten U.S. bases in Greenland. While the United States has mostly focused on rebuilding Alaska's defenses, Greenland has not been forgotten. According to The Drive, the Thule Airbase is tasked with protecting against nuclear attack while also ensuring that Russian aircraft do not attack the northern United States from the Arctic. In the event of an invasion, Greenland is the first line of defense of the North American continent before matters reach the U.S. mainland.
Alaska
Greenland is half of the U.S. Arctic defense strategy. The other half is Alaska, which holds the title as the only piece of U.S. soil to be occupied by a foreign army since the War of 1812. According to American Experience, Japanese forces invaded the Aleutian islands and attempted to set up military bases there that would block U.S. expansion in the Pacific. Americans, however, also feared that it would serve as a springboard for strikes against, or even an invasion of, the lower 48. The Japanese invasion showcased Alaska's strategic value.
This barren territory had originally been dubbed "Seward's Folly," and was poorly defended. During the Cold War, the proximity to the USSR lent greater strategic importance to the Last Frontier, which today serves as a major lynchpin of the United States' strategy in the Arctic.
Today, Alaska is again at the forefront of another global rivalry, this time between the United States and China. According to Arctic Today, in September 2021, the U.S. Coast Guard encountered Chinese warships near the Aleutian Islands. In response, the United States military has continued to build up troop presence in Alaska, according to Business Insider. Naturally, this would make a powerful deterrent against attempts to repeat the Japanese invasion. However, there does not seem to be any fear of invasion, at least not yet. Chinese forces appear more interested in access to the Arctic via the Bering Strait for now, but the buildup of forces suggests that the Chinese presence may have brought up memories of 1942.
The Second Amendment
If a foreign army were to make it to U.S. soil, the single biggest defense would be the Second Amendment of the United States Constitution. According to the Constitution Center, the Second Amendment was originally written to prevent federal overreach upon citizens rights. This armed citizen militia was not only capable of checking federal overreach through organs of the state. It also served as an ad-hoc army that could quickly be raised in the event of an armed invasion of the mainland.
Although times have changed, the amendment still serves this purpose. Forbes has noted that the maintenance of a "well regulated" (meaning "well trained and equipped") militia serves as a cheap deterrent to foreign invasion. According to WAMU, American civilians, many of whom include combat veterans, own around 400 million arms of varying grades. Combined with the sheer size of the United States, any foreign army would undoubtedly have to contend with militias harassing it throughout rough, difficult terrain such as Appalachia, the Rockies, or the wide-open Great Plains. Although politicians and activists have called for the Second Amendment's repeal, Forbes notes that an armed society is a safe society and a cheap deterrent against foreign invasion that the United States would be foolish to give up.
An invasion is not likely
All of the above coupled with the immense size of the United States makes the country virtually impossible to invade, according to Vice. When questioned about an invasion of the country, AAFA analyst Dylan Lehrke noted that such a concept is virtually impossible even on paper (via Vice). Even if the nuclear arsenal were somehow disabled, which would already be difficult due to its diffusion among different military branches, the country is simply too big. Any invasion would require basing in Canada or Mexico and then fighting tooth and nail just to gain a foothold on the mainland, whether overland or through amphibious landings.
Ultimately, Lehrke notes that the U.S. military is well positioned to deflect land invasions. Armored divisions are stationed in Texas for exactly this purpose. Invasions through Canada would run into the problem of rough, forested terrain and numerous lakes that would make deploying anything more than light infantry a logistical impossibility. And this is assuming that the invaders manage to overcome the Atlantic and Pacific oceans, which Lehrke argues place enemy forces at the mercy of U.S. naval and air power. However, Lehrke does mention that any war against the United States would have to be non-conventional, including but not limited to cyber and economic warfare.
The soft underbelly
The southern border with Mexico, according to Vice, is probably the easiest place to invade the United States. The idea of Mexico as a staging ground for invasion is not new. The 1917 Zimmerman Telegram from Imperial Germany urged Mexico to attack the United States while forces were occupied in Europe. Now, it is unlikely that Mexico would invade the United States. Mexico is the U.S.' second biggest trading partner and U.S. businesses have too much to lose for this relationship to change.
However, there is the problem of non-conventional warfare against paramilitary groups, weaponized migration, and cartels. Some conservative critics argue that the migrant flows at the southern border are an invasion in themselves (via the Imaginative Conservative). However, the Border Report notes that migrant flows are the sign of a much bigger problem: Mexico's violent and ruthless drug cartels.
Analyst Sylvia Longmire's "Cartel: The Coming Invasion of Mexico's Drug Wars" predicts a cartel invasion of the U.S. rather than a Mexican one. Cartels have taken over sections of U.S. cities virtually unopposed, according to the Washington Post. Although not an invasion in the traditional sense, Chron notes that the effects of this cartel "invasion" have served to spike opioid usage and destroy middle America's tight-knit towns and suburban communities. This type of silent invasion, in fact, may be much deadlier than a conventional one, as it is harder to stop.
UN occupation
The other way the United States could theoretically fall to foreign invasion would be in the case of civil unrest. According to the Military Times, the Obama administration ratified the Kigali Principles, pledging to support the use of U.N. peacekeepers anywhere in the world where civilians are threatened by civil strife. Were the U.S. to experience any sort of unrest or political instability, this in theory could trigger a U.N. invasion of the country. The New York Times notes that since the 1950s, human rights campaigners and civil rights leaders have requested U.N. investigations into the United States' human rights record, although none of them called for armed intervention in the United States.
While the New York Times was suggesting the need for U.N. oversight into the 2020 presidential election, such intervention would likely come in the form of a condemnation, not armed occupation. But if the situation were to escalate to violence, it is easy to see how the possibility of U.N. armed intervention could materialize under the Kigali Principles if American civilians were placed in harm's way. Following the pattern of other conflicts, such as the Syrian War, it is likely that foreign countries would pact with rebel factions and place their own boots on the ground too if possible.
An alien invasion
So far, all of these invasion scenarios have dealt with human attacks against the United States. But what about an invasion of little green humanoids armed with laser blasters and hyperspace technology? This may not be a major worry for anyone outside conspiracy aficionados, but two Defense Department contractors have taken this threat seriously enough to prepare a blueprint to defend against it.
"An Introduction to Planetary Defense" is exactly the kind of book that makes conspiracy aficionados feel vindicated. It not only takes the idea of an extraterrestrial invasion seriously, it provides a framework to counter it at the planetary level, suggesting that the world would have to fight back together. According to Reuters, the world would ideally prepare for an alien invasion. But since that is not likely, the authors suggest that civilians would have to mount a guerrilla campaign, wearing down alien invaders until they finally decide to leave Earth.
The Washington Post has suggested that this entire problem could be avoided by simply not trying to contact aliens. Any alien life out there would likely be more advanced than any civilization on earth due to older solar systems and having more time to develop. Thus, an alien invasion would have a high probability of overrunning Earth and ending human life. So why even take the risk? In the end, it may be too late. If the federal government really is hiding aliens in Area 51, it might be time to read up on planetary defense and prepare accordingly.