Actually, the Korean, Vietnam and all the cold war related skirmishes are related to defense. The soviets were actively engaged in subverting governments and taking them over and we were the great big prize target for them. When you picture the scenario i describe and think to yourself "there is no way the threat the soviets posed to us was that sinister and involved," know that in fact i was that sinister and convoluted and involved. The soviets were truly trying to take over the world in such a manner that if one understood it they would no longer believe in any way that the US had ever made any attempt to do the same.
The war in Iraq and Afghanistan are related to defense as well. Iraqi intelligence has been giving aid to Al Qaeda since the early 90's as they have waged a war against us. The Afghanistan Taliban gave them safe haven from which to plan and conduct this war. Is that enough reason to go to war? That's were the debate starts, but to say that there has been no provocation on any level since WWII is naive. Though i can see claiming that since a lot of it has not been direct provocation it doesn't count, but i would disagree on that point.
Since the end of World War II the US has faced some form of constant threat. Whether or not these threats have been severe enough to warrant the actions we have taken is a separate discussion, but the threats have been there. Those parties that have threatened us have always tried to conduct themselves in such a way that they could always claim any retaliation was unwarranted.
All in all, I think we should have given up Global Super Power status when the soviets fell. It's just not worth it anymore, but I think we were needed while they were standing.
We should stop bothering with foreign entanglements, but there are a lot of people who aren't going to like the fact that that would mean no help for Darfur, or Haiti, no more aid like we gave after the tsunami.
Soviet stuff: http://www.metacafe.com/watch/yt-RS8LA-5fmrs/g_edward_griffin_interview_of_yuri_bezmenov/
Yuri Bezmenov was one of the guys doing it until he defected. There is also a series of 7 videos where he gives a university lecture and goes into detail on this process. He is wearing a powder blue blazer in that series.
I know people want to believe the US is some big bad boogey man ruining the lives of innocent foreigners, and we have done some really nasty things. But if you look back at all of the nations that have been the big superpower through history, the US really does set the gold standard for benevolence in global politics.