When should America (or any country) go to war?

Outside of a direct attack on our soil, when should America invade another country?

Because they shot down a drone? Because they’re compromising trade? Because their leader is oppressing his people? This isn’t just about Iran, though of course that is what has gotten me thinking about this.

When is it just justified to launch a war that is not the result of a direct attack on our homeland?

While this is mainly aimed Americans, non-Americans can answer for their own country too.