Then came the horror of it all and Europe's upper classes had their youngest and brightest massacred and values changed. Warriors were now ordinary suckers and the elites kept themselves above the churn. War became a business of calculated leverage. It on that massive change in context that we changed together to deciding that war is not always such a good idea.
Then on top of that conflicts like those Vietnam and Afghanistan merely emphasize the baseness and poor value return of armed conflict. What was a sure thing before WWI was a daring bet in the nuclear age an nukes were only one factor involved in that calculation.