Great idea

The only problem with this is that you'd have an army filled with people who could be a hinderance more than benefit to the army (like old people), great idea in theory though

Why are we not doing this?

Were this implemented we would be speaking Japanese now because we would have been invaded before we could have scheduled and held a vote.

*in the cold war*
Russia: Oh look, America has an amendment that says they cant take military action unless every citizen votes for it. *Drops every single nuke they have on USA*
President: Oh no, Russia is nuking us
Advisor: well bc of that stupid amendment we cant do shit unless we call a vote lol
President: Quick call a vo- *BOOM*
Yeah great idea.