[amysox]

Robert Heinlein proposed something like that in his "lost" novel For Us, the Living: instead of declaring war, Congress would authorize a war referendum, in which only those eligible for military service could vote. The catch was, everyone voting "Yes" would thereby automatically sign themselves up in the military for the duration. If a further draft was needed, it would first be composed of those that didn't vote, and lastly those who voted "No."

(In the book, the "future" Congress had called war referenda on three occasions; each time, the vote was overwhelmingly "No," and historians believed those decisions were justified.)

[OkayPhysicist]

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.

[Arodex]

It seems a good idea, but then I remember the history of World War I: the French and the German populations were very happy and willing to go to war (especially the French). A quick war that will settle it, they thought. And then they ended up in one of the worst meatgrinders of history.

Even if they had a referendum system such as the one proposed, I don't think that would make it a better decision. And given how people don't assume the consequences of their decisions, they would just find excuses later on - "we were manipulated, they lied to us" - but the damage will still be done.