system
Given the following post, respond as if you're commenting on Reddit:
human
ELI5: Are there any countries in the world that can realistically invade the US by force?

By air and sea like a full on invasion where we'd be seeing some serious shit. Or even like an event leading up to an invasion like the bombing of a coast or capture of a major port or something like that..  Just wondering if like Russia or China might have the capability to do this. Thanks!

Edit: holy responses! It's cool reading through every response and seeing a different scenarios! Didn't think about the whole housing/financial market collapse or cyberspace attack! Crazy stuff... And no I'm not a terrorist!
gpt
No. The USA shares land borders with two countries, neither of which have military assets comparable to the USA -- the USA spends $682 billion, to Canada's $22 billion and Mexico's $6 billion. The only countries with military expenditures in arguably the same ballpark are Russia and China.

For countries that don't share a border, an invasion would have to come by sea. To project force overseas a nation requires a blue water navy and aircraft carriers. The USA has 10 aircraft carriers (plus 2 in reserve); there are perhaps 10 other countries that have 1-2 carriers each. The difference between an American carrier and a foreign carrier, just in terms of size and displacement to say nothing of armament, personnel, or training, is staggering -- the UK's (a nation known for its navy) last carrier displaced ~20,000 tons, USA's carriers displace over 100,000 tons. 

tldr; Russia and China lack a significant blue water navy. They have no way of effectively projecting force into North America.