It would help if those who start a war are required to spend a portion of their time at the front lines. See how fast these boomers walk back their armies.

Not sure about that though. For a large portion of the (european) middle ages it was common and even expected from rulers to fight in battle. It’s not like that was an exceptionally peaceful time (nor was it exceptionally warlike though). Yet it should be mentioned that high ranking combatants like kings, knights and other nobles could expect to be captured and ransomed instead of being killed. But still, it was a risk and many a ruler were killed in battle.