I got into the anti-war and anti-globalization movement as a clueless college student in the early 2000s. I dropped out after a year or so because it felt like it was going nowhere. In retrospect, it was because we didn't have anything to fight against -- remember, this was before 9/11 and the War on Terrorism. Even the invasion of Afghanistan and the later occupation of Iraq didn't feel as monumentally disastrous as the Vietnam War or our propping up banana republics in Latin America or playing Iran and Iraq against each other.
At the time I'd listened to recordings of people who were railing against our intervention on the Balkans in the late '90s, but that too never struck me as what you would've heard standing around the Columbia student union in 1968 or hearing Noam Chomsky speak about Guatemala in the 1980s. That feeling only stuck with me through the 2000s. Had U.S. imperialism shriveled into something barely worth the time to march against?
Fortunately, unlike most large-scale changes that went into effect starting in the mid-1990s, there is no lame-ass story for people to tell here about how the internet changed everything. Whenever I see a huge social or institutional change that took place then, I immediately think of the falling crime rates and the larger domestication of society. (That's also when the sexual counter-revolution began, i.e. when promiscuity started falling, and when rock music died off.) The good thing here is that we have data that go back over a century on homicide rates. Plus there were actually two crime waves during the 20th C., which gives us four eras to test our ideas against (two rising-crime eras and two falling-crime eras). Non-obvious theories that talk about the internet are mostly untestable because we haven't seen the internet cycle that much. Any change that took place in the '90s you can lazily spin a story about how the internet did it.
The rising-crime eras were at least 1900 (and maybe a bit further back, but this is where the data begin) through 1933, and again from 1959 through 1991. The falling-crime eras are from 1934 through 1958 and again from 1992 to present. To check how militaristic the U.S. has been during these periods, there may be some fancy index of interventionism that a political scientist knows of, but just have a quick look at the Wikipedia lists of U.S. military operations and U.S. covert overseas regime change. All cases that fall in the rising-crime eras fit my hypothesis, and indeed they seem to be the worst, such as our involvement in the Pacific Islands, Latin America, and the Caribbean during the early 20th C, the wars in Southeast Asia, our involvement in Latin America in the '70s and '80s, and our involvement in the Middle East in the '60s through the early '90s.
So what are the counter-examples from the falling-crime eras? For covert regime change, there are two big exceptions: overthrowing Mossadegh in Iran (1953) and Arbenz in Guatemala (1954). The cases from 1992 and after don't appear as well established and certainly were not on the scale of the previously mentioned two. Ditto for the cases of merely supporting dissidents in Communist countries during the 1940s. The facts fit pretty well.
What about the list of military operations? Pretty much the same thing. There's a lot going on in the Pacific Island and the Caribbean around the turn of the last century, and imperialism stays pretty high through the 1910s and '20s. Then look at the 1930s -- basically nothing, certainly not from 1934 onward. Incidentally, the U.S. occupied Haiti from 1915 to 1934 -- exiting the very first year of the falling-crime era. There's essentially nothing in the 1940s either, other than WWII. But that's not a case of the U.S. meddling in factional violence in some unimportant part of the world, or pushing around local puppets to get better access to natural resources or just show the other loser countries who's boss. That's not imperialism. Neither was the Korean War for that matter.
There are a few operations unrelated to Korea in the 1950s, but again hardly any and definitely not on the scale of occupying Haiti, invading Cuba, etc. The U.S. does begin to send troops into Southeast Asia toward the end of this falling-crime era, so that's one possible counter-example, although those wars did not ramp up until Kennedy came into office during a rising-crime era. All of the operations from the Cuban Missle Crisis of 1962 through the Gulf War of 1991 took place during rising-crime times as well.
As for the operations during the falling-crime era of 1992 through today, again just look and find me something that compares to what we saw in Argentina, Brazil, El Salvador, Iran and Iraq, Vietnam, etc. There's basically jackshit aside from the War in Afghanistan and the occupation of Iraq, but the first ended very quickly and the second is still not close to our 20-year occupation of Haiti (though maybe I'll have to come back and change this many years from now).
It's not a perfect fit, but what the hell, it's only social science. It matches up better than any other theory of imperialist aggression. Note that macro-level political and economic changes don't have anything to do with it. We were remarkably non-interventionist during the 1930s -- and of course that had to be due to the Great Depression! Just like how our current recession is keeping us focused on more important things than invading another sandbox. Except that we had a very severe recession in the early 1920s, another bad one in the mid-'70s, a terrible one in the early '80s, and a fairly painful one in the early '90s, all when we were heavily interventionist. Capitalism, the free market, GDP, standard-of-living, etc. -- all of that was steadily rising over the 20th century through today, yet there are major swings both up and down in interventionism.
Politics has grown steadily more competitive and responsive to public opinion, so again that can't account for the two major downswings in interventionism (or upswings, depending on how you thought politics influenced foreign policy). Party affiliation of leaders doesn't matter because Kennedy and LBJ were just as interventionist as Nixon and Regan or Theodore Roosevelt and Wilson. Likewise FDR and Eisenhower were just as restrained as Clinton and Bush II (again, on a relative level).
The only political variable that foreign interventionism tracks pretty well is the politicians focus on fighting crime at home. Politicians can't generally get away with something that the majority of people won't tolerate (see Bryan Caplan's The Myth of the Rational Voter), so we infer that average citizens were fairly tolerant of aggressive foreign policies during rising-crime eras. Well sure they were -- they even demanded it in their popular culture! What great action flick from the '80s did not feature a banana republic? We also infer that when crime rates start falling, the public senses this and is no longer tolerant of blunderbuss approaches to foreign policy -- things seem safer and safer at home, so we believe that foreign threats to our safety must be dropping too, unless we have clear evidence to the contrary like 9/11.
To sum up the process, when crime rates start rising, the public senses this in their own life and through word-of-mouth (plus hearing it from the media). They want the authorities to crack down on dangerous people before they even get the chance to start running amok. But why stop the logic at the local border? We all know there are people abroad who might pose a threat to us, and since we can't tell that from personal experience, we just have to trust what the experts say about foreign threats -- and they mostly pander to our fear of rising crime at home. Hey, once those Vietnamese farmers organize a communist government, they're only a hop skip and a jump away from Columbus, Ohio! So sure, the public says, go get them. And then send in G.I. Joe to fuck up the Sandinistas. Politicians respond to the public's perception of rising danger by getting tough on crime at home and by staging more and larger-scale military operations abroad.
Once crime rates start falling, this process unwinds. The public senses that things are safer and safer, so they are less in a state of panic. They believe we shouldn't invest so much in tough-on-crime measures since the problem appears to be taking care of itself somehow. And with no fear to exploit, foreign policy hawks can't whip us into a fear of Grenada or Afghanistan. (Again, there could be an exceptional shock like 9/11, and then hawks can get the public on board to occupy a sandbox for awhile.) Politicians respond to this change in voter sentiment by not ranting about getting tough on crime so much anymore and by starting to dial down how interventionist their policies are.
Basically, there's a threat of danger out there and it doesn't matter whether the public perceives it coming from local thieves and murderers or foreign guerrillas. There are just bad guys out there somewhere. When this threat level appears to be rising, voters demand more militaristic responses from politicians, again without respect to national borders. And not wanting to get voted out of office, the politicians respond by getting tough on crime and by bombing nobody countries back into the stone age or getting in bed with local thugs who will crack down on subversives. Once voters perceive the threat level to be receding, everything reverses course. No other theory I've ever heard of can account for the decades-long stretches of imperialism that are then interrupted by decades of relative isolationism, only to be followed by several more decades of imperialism, which in turn are put to rest by more decades still of isolationism.