From the Cool Ideas That Would Be Impossible To Implement file:

Countries should decide whether to get involved in any particular military action through a secret-ballot referendum of all the members of its armed forces. The members of the armed forces are given all the information available, even that which is not available to the general public (because that's what security clearance is for), and then they decide for themselves whether it's a worthwhile cause.

Possible variation: votes are weighted based on how close to the action the voter would be (although that does take away some of the secrecy of the secret ballot). So the votes of people whose job is to be cannon fodder would be worth more than the votes of people whose job is to sit in an office in another continent.