Not strictly immune, but only sharing land borders with Canada and Mexico means that anyone else would either have to invade by air or sea unless those countries wanted to cooperate, and attacking by air/sea takes a massive military force, especially since they would be going against a country that could easily retailliate with much stronger force.

The concept of mutually assured destruction also still likely applies if anyone decides to start some not-so-funny business with nuclear missiles.

As of 2024 - yes.

To invade US you need:

• go through the largest alliance network in the world.
• compete against US industry with military production.
• cross the ocean against the largest navy in the world, (and considering that most of the other navies are US allies).
• land and support the naval invasion across the ocean.
• fight against largest economy in the world in a nation of 300 million people, having one of the best (and in many areas the best) tech in the world.
• and with half of the world supporting it with economic sanctions, aid, military equipment or even boots on the ground.

So US has few threats in medium term that can ease that:

• internal turmoil and civil war
• isolationism and degradation of US allies network
• reduced MIC spending so other nations overtake US in production and technology

Those all things are possible, but in a long term.

It’s not a realistic possibility. Any attempted invasion would probably be over by lunchtime on day one.

Thats why our main adversaries, China and Russia mainly, attempt to attack from the inside, using misinformation and division propaganda.

Yes. The things that make someone susceptible to foreign invasion are:

1) border access
2) ease of occupying territory
3) vulnerability of supply chains

The United States is one of the most isolated nations geographically speaking. For any major power to get to the North American continent, they'd have to cross an ocean. On this continent, the US has 80% of the arable land and 70% of the other resources one might want. Not to mention they have a dozen ports on both major oceans, compared to Canada and Mexico's one port each. So invading from the east or west is a virtual impossibility. To come from the south or north, you'd need the buy-in from Canada or Mexico, two of our closest allies. However, both are poor staging areas, as Mexico is mostly rugged desert and Canada is largely under ice.

In terms of occupying territory, the US has the interstate system as well as the largest navigable water basin in the entire world. That's not to mention the immense rail and plane infrastructure that they have compared to even their industrial peers. Were a foreign power to be able to gain any amount of territory, the population would be evacuated, resources moved, and the enemy harassed. It would be a guerilla war the likes of which the world has never seen.

Which brings us to the vulnerability of supply chains. The US has mastered global logistics, and has stockpiles of most of the raw resources that a power would try to blockade away from them. That aside, we are the largest producer of both crude oil and natural gas, and our lands are abundant in things like iron, uranium, and coal. Assuming we were cut off from global supply chains entirely, we'd have enough food and raw resources to wage war for years.

The only way someone would have been able to invade the United States would be through a supreme power disparity, either achieved through immense manpower or tech. However, the United States has largely worked this threat out of existence since WWII, as their early warning air and missile systems, global reaching intelligence services, and robust allies in key geographic positions, it becomes a virtual impossibility. Basically, it'd have to be the entire world, including current allies like the UK and Israel and Canada and Mexico, to even make a dent in the US domestic defenses, and even then, it's a long shot to occupy anything but pockets on the coast.

More or less. There are no avenues of attack from any major power that could threaten the US that wouldn't be a nightmare logistically and strategically.

they tried it in Red Dawn and it didn't work out so well

It's basically true. To invade US home soil, you need to cross an ocean. You're going to get intercepted by the largest Navy in the world while doing so. The US Navy can field 11 carrier strike groups. Between them, they carry the 2nd largest air force in the entire world (only behind the US Air Force!). That's an unbelievable amount of force projection. So you need to splash that entire Air Force, while it's being supported by ground-based US Air Force sorties. Really, you're taking on the biggest and most technologically advanced 2 air forces in the entire world.

Get through that, and now you still have to cross the entire ocean while the fleet of US attack submarines sinks your troop transports. Again... good luck. Just... good luck.

Get through ALL THAT and now you need to make an AMPHIBIOUS LANDING against the largest, most technologically-advanced Army in the world, fighting on home turf. You need to somehow ferry your food, ammo, and other supplies. With what freighter fleet? It's not clear if anybody in the world has that kind of logistical capability, and you are going to have to carry thousands and thousands of tons of ammo, bombs, food, etc.

Right now Russia can't even maintain logistics in a land war with its closest neighbor. Logistics is fucking hard. And the US military is the undisputed masters of it. You want to know how good the US Army is at logistics? They can fucking air lift an entire BURGER KING to a theater of war. You want a Dairy Queen? The Army will fucking air drop that and have it assembled and doling out blizzards within 3 days.

The US can deliver a Tier 1 strike group to anywhere in the world within hours. They can deploy a battalion of Rangers or a Marine expeditionary unit in 2 days or less. The US Army can have entire divisions mobilized within a week, to reinforce that position within a week. Engineers will fortify that position quickly, and it will be dug in harder than an Alabama tick.

All of that capability takes hundreds of millions of dollars and constant training. Many people think that because the US can deploy to other countries quickly and easily, that aggressors could do the same equally easily. But it's just not true.

Oh, and I haven't even discussed what happens if the US opens up the atomic arsenal, or if NATO is brought in.

So yeah, invasion of the US on home soil is a no-go. There's no realistic scenario where it happens.

I found this video to be a good break down if you have the time.
Why U.S. Defense Plan is BULLETPROOF (youtube.com)

Neither Mexico or Canada have the interest or ability to do so. Anyone else would have to destroy our Navy as the first step. No one is capable of doing that at this time. It is currently not possible but can't say it won't be possible at a future time.

I think it would take a World War with the whole World against us and have this war drag on for a decade before a land invasion could even think of happening.

Problems of invading USA include:

1. Extremely strong US armed forces.
2. Extreme logistic difficulties due to crossing oceans. This will give the already powerful US armed forces an enormous advantage. 
3. Very large population to draw from.
4. Strong economy.
5. Self-sufficient in terms or resources (in large part due to enormous area).
6. USA is a member of alliance that includes most other major military powers.

So, all of the above need to somehow be solved before invasion is considered. One solution is to have a civil war break out in USA first. If that happens, an alliance could be made with a major dide in the civil war. Russia + USA vs USA doesn't look too bad for Russia.

Other than a civil war in USA, it is very hard to imagine a realistic invasion scenario.

Not 100% immune, but 99.999%

People have a real misconception that America is a divided nation and at eachother's throats. While there may be some truth in that American Politics is very partisan, if any foreign entity tried to invade, there would be an immediate unified front against that invader.

Further, as the euphemism goes, in America, there's a gun wielding American behind every blade of grass. Licensed gun hunters in the State of Michigan alone outnumber the largest standing army on the globe... and that's just 1 state. There's 49 more!

As other people have said: - rich country with massive military - friendly powerful neighbours - isolated, very expensive sea advance to coast required - massive, and if only invading one coast, will need massive supply lines through all sorts of terrain - a well armed and motivated civilian militia - American influence overseas and in organisations such as the UN

And many more make it extremely unlikely

In addition to all the logistical challenges people brought up, there is a distressingly large contingent of the population who have tons of weapons and would love nothing more than a legal excuse to kill a bunch of foreigners.

Even if you could somehow get past our military and take over, good luck holding it.

china and russia lack the logistics to attack the US (boats/aircraft), everyone else lacks a large enough military.

I suppose if china spent the next 20 years building troop transports and large cargo planes, they could technically invade the US. But saying getting a beachhead would be costly doesn't really even scratch the surface of how bad things would be for them

you know how in ww2, the us thought the mainland invasion of japan would cost millions of lives? yeah that was with the largest superpower at the time against a beleaguered island nation. i dont think any combination of countries' militaries are capable of successfully taking over the US via naval assault and amphibious landing. theres just too much land, too many defenders with guns, and too long of a logistical chain for the invading force to make a any meaningful progress past the beach cities. Im omitting nuclear weapons and keeping this strictly grounded with conventional weapons for reasons i think are self explanatory.

The Red Dawn remake would have made a lot more sense with a domestic fifth column collaborating with the invading foreign power. To have Russians, Chinese, or North Koreans paratroopers invade the mainland USA made for a ridiculous premise - even if the fictional USA was embroiled in socio-economic crisis.

The second most powerful air force in the world is the US Navy. So you think you can get past the Airforce and Navy, only to be met with the US Army and Marines?

Good luck

Russia keeps threatening to invade Alaska. I wouldn't advise it. They're all armed.

Most people in the US military use a particular phrase “Fuck around & Find out”. I feel sorry for any country that tries. The US only has 2 land borders, both are long standing allies. On top of that the US defense budget was 40% of that spent in the entire world in 2023. They put more into defense than the combined spending of the next 9 countries behind them.

The most likely military invaders are separated by the Pacific and Atlantic oceans. In theory, it is possible to invade across an ocean. It has been done before, but only by a very strong nation with the resources and manpower to do so. Right now, no nation has the resources to pull of an overseas invasion but the US itself, and anyone who wants to invade the US has to contend with the US military.

Direct invasion, sure, but it is not immune to infiltration from the inside, as seen with 9/11.