It probably started much sooner than the beginning of the 21st century. I think many Americans saw it as far back as the 1960s and during the 1970s. They saw it when the Pentagon Papers story broke, as well as other revelations about the CIA, FBI, NSA, and other arms of the government came to light. As a consequence, there arose a strong undercurrent in public opinion which held a great deal of mistrust of politicians and government in general - something that has remained to the present day.
But the government has clearly countered such attacks and fought back against its accusers and detractors, which happened during the 1980s and 90s. They were mostly successful in shifting public opinion away from any kind of anti-war/anti-military sentiment, particularly by the time of the first Gulf War in 1990-91, when "support the troops" became the catchphrase of the day. I saw people I knew who were against US involvement in Vietnam do a complete 180 and fervently supported Operation Desert Storm.
Sometimes, truth is revealed in comedy:
I think public opinion has fluctuated on this point over the years. World War 2 was considered the "good war," and public perceptions regarding that war have influenced opinion regarding war in general, particularly the concept of a "just war." I've heard it said that, without Hitler, all these right-wing warmongers would be lost today. I remember Phil Donahue cleverly said in that context, "Hitler gave a good name to war." Any time someone wants to go to war, they seemingly conjure up the ghost of Hitler to remind and warn people of the danger they're in from the evil around the world.
It goes back to the notion of "Appeasement," which was viewed as a grave blunder on the part of the Allies. It led to the idea that if we don't stand up to [insert name of enemy] now, then it will be seen as weakness and will only encourage more aggressiveness, just like Hitler. That mentality was what influenced the policy of containment, rooted in the belief that communists must be met with force wherever they spring up.
They said that if we didn't stand up to the communists in Korea, they would just keep going, into Japan, across the Pacific, and into the U.S. They said that if we didn't stand up to the communists in Vietnam, the rest of Southeast Asia would fall like dominoes, according to the infamous "Domino Theory." Cuba was another sore point and had to be "quarantined," and when Nicaragua was taken over by the Sandinistas, a variation of the same Domino Theory was propagated, except in Central America, Mexico, then the U.S. (which Hollywood borrowed to write the script for "Red Dawn").
One thing that came out of the Vietnam era, at least from the militaristic side, was the belief that the reasons for fighting the war were not necessarily wrong, but it was the way it was fought that was wrong. Reagan, for example, criticized the fact that we sent troops to Vietnam, but their hands were tied and they were "not allowed to win." It echoes a line attributed to Patton: "The politicians never let us finish. They always stop short and leave us with another war."
That's kind of what they did in 1991 in Iraq. They stopped short and didn't finish. So, then they went back in 2003 to finish off Hussein. But they still didn't finish it, since Iraq was never fully pacified or under total US control. And Afghanistan was another debacle.
It's one thing to be a warmonger, but to be a warmonger who does not want to win the wars they're in - that's truly bizarre.
