The only acceptable act of war is an act of defense.
This means that we should only ever use military force if an entity attacks us directly, tries to attack us directly, or invades our territory directly.
The threat or suspicion of an attack is not enough. We must have hard evidence to support our actions. Otherwise we risk creating the enemy we are trying to eliminate.
This applies even more strictly to the aid we give to our allies. We should never join the wars of our allies unless they are in danger of being overrun by their enemies, as was the case in the World Wars.
Innocent people should not have to die because of someone else's greed, fear, and hatred.