Simpler: Leaders who engage in military engagements should face a public referendum 6 months/1 year later, with exactly 1 question: "Should [Leader] die?". Presidents in the US rarely get elected with less than 40% of the popular vote, and I reckon about 30% of people wouldn't vote to kill someone pretty much no matter what they did. Half of those people are your supporters, so really it's just a matter of avoiding your popularity plummeting to sub 35% because of your warmongering. By that math. By that math, Trump, W. Bush, and Truman would have gotten the gallows. Biden, Carter, and Nixon all fell below 35%, but didn't start any foreign wars.