> I'm not entirely certain of my history, so here's as good a place to ask as any: when was the last time the United States was under threat of invasion from aggressive foreign powers?

Most recently probably 1979 to 1985:

http://en.wikipedia.org/wiki/Cold_war#Late_1970s_deteriorati...

Threat of even a remote chance of attempted invasion/occupation ending 1989-1991 with the dissolution of the USSR.

That said, massive global conflicts don't start with the superpower getting invaded - they start with the aggressor seizing control of their local economies, conscripting soldiers, and invading for strategic locations and resources near them.

I was just talking about this with a friend of mine - what would the world look like without America as the world police? Any time a medium sized country is thinking of invading a smaller country, they need to consider whether the United States will intervene.

So for instance, think about the first Gulf War. Saddam's Iraq invaded Kuwait. Map here:

http://i.infoplease.com/images/mapmiddleeast.gif

Lots of oil in Kuwait, small and weak military compared to Iraq of that time, strategic position in the Persian Gulf, etc.

Let's say America and allies doesn't step up and stop him. He conquers and assimilates Kuwait. What's next? Invade Saudi Arabia for Mecca and trying to install himself or a puppet actor as a new Caliph (supreme Muslim leader, office dissolved under secular Turkey post-WWI)? That'd give him legitimacy under an Islamic war banner. Try to take over Iran? Semi-peaceful assimilation of another neighbor? "Holy" war on Israel?

Certainly - Kuwait wasn't an end-all, be-all goal for Saddam. Who knows what he'd have done if the UN coalition led by America hadn't stopped him?

I mention these things because I'm generally in favor of American non-interventionist policies. American world policing is very expensive. The American people have had the benefit of living in the world's dominant empire recently, the standard global currency, the generally accepted world political system, the most military bases across the world, a fairly high quality passport, but at this point, the cost is probably higher than its worth for its citizens, and the job America does is a mixed bag for the rest of the world.

It's also kind of capricious in where it intervenes - there was no real American interest in the Kosovo campaign, whereas in Burma horrible things are happening similar to what happened in Kosovo. Why Kosovo, which was expensive, no strategic value to the USA, and no threat to the USA, but not Burma, which would also be expensive, have no strategic value to the USA, and is no threat to the USA? Similar sorts of violence/repression/abuses happening in both places.

Alright, that was a bit tangential. So to answer your question, America is absolutely in no danger of being invaded, what with the world's most powerful military and the geography layout of the world. But imagine if America went non-interventionist, or decommissioned its Navy and went to a a much cheaper homeland defense urban skirmishing style of military - how long would it be until North Korea went to "unify" with South Korea, or cooked up a pretense to invade Japan? I'd guess pretty fast. And that's how aggressive powers are built - they don't take on the world's dominant power until they've seized control over home industry, begun conscripting, and started taking over strategic places near them.