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World War 1.
At the beginning of that war, everyone worth a shit rushed in to join together to fight for God & country. Once they experienced the reality of tens of thousands of people at a time getting mowed down senselessly by machinery without even having the chance to put up a fight, they realized there was no glory to be had. ...a lesson reiterated in every war since, making war much more unlikely & proportional.
The US, however, has found a way around this, even after the disasters of its war efforts in Korea, South America, and Vietnam by not requiring compulsory military service, separating the decision to go to war from the people who will actually go to war, thus ensuring a continual state of more and crazier wars until it is destroyed.