If everyone outside the US suddenly decided to attack it, would they win?

I think this is a really interesting question. Without a doubt, America’s military is far more powerful than any military in the world. However, America obviously has a much smaller military than the COMBINED rest of the world. Still, I don’t think America would lose on this one. Here’s why:

1. Most importantly, attacking the United States from overseas is not the same as attacking it from the land. Sure, China and Russia have large militaries, but just how would you get them to America? The combined navy in the rest of the world probably actually is less powerful than the United States Navy. Even if the U.S. Navy didn’t exist, Russia and China don’t have the ability to transport large-enough troops, heavy equipment, aircraft, fuel, support, etc. overseas to defeat the United States, which would only have to move those things within its borders for defense. This is even more true for the smaller countries, which don’t have much capability of fighting far away from their own borders. Russia’s one of the big dogs and it has, what, maybe 50 landing ships, most of which carry less than a hundred people? What are you even going to do with that? China’s even worse off, so how would they even hope to begin sending their huge army to the USA? And remember – most countries don’t have *any* landing ships.
2. Most of the rest of the world,including the big powers (Russia and China) have rusty old militaries from the 1980s and (much) older. The United States has far superior tanks, planes, etc. Russia may have a lot of tanks, but most of them are practically antiques. I doubt five of their awful T-62s could stand up to a single Abrams. And then, of course, there’s the problem of actually bringing in enough tanks from overseas on ships to actually fight America’s 10,000 Abrams tanks. See point number 1.
3. Even if the rest of the world’s miltaries eventually wore down the US military, then you’d have the added problem of still fighting against the largest armed populace on the planet – the citizens of the United States. Don’t forget that there are 300 million citizens in the United States, and they are in possession of 350 million guns, and an unbelievable amount of ammunition (12 billion rounds of ammunition are sold to American citizens every year). Many of those guns are military-style small arms, and tens of millions of the citizens are military veterans. Assuming that 200 million of those citizens would actually be capable of picking up a gun, what remaining shreds of foreign military is able to haul enough men and resources from around the world to fight a guerrilla army of an astonishing 200 million people?

So no, I don’t think the rest of the world could win, short of nuclear war. However, a nuclear war would be silly, as it would leave the world uninhabitable. With all the nuclear silos secretly spread out all over the United States, I don’t think you could wipe them out before launch.

I suppose you could try to cut off the U.S. oil supply, but most of USA oil comes from either the United States, Mexico, or Canada, and since those countries are neighboring and with relatively small militaries, I imagine the USA could grab those oil sources pretty quickly.

Again, if America was in central Asia or something, the story would be different. But America is essentially a geographical island unto itself, and that means transporting the war overseas, which makes things tremendously complicated and resource-needy.