Is there EVER a valid reason for a country to go to war? [View All]

This spirited conversation got me wondering, aside from a country being directly attacked by another, is there ever valid justification for a nation to go to war?

The rise of a global menace like Hitler? Brutal human rights violations and war crimes, such as the Holocaust? Neither of these situations involved the US being attacked by the Germans but I doubt anyone would seriously say that stopping the Nazis was wrong. If we had not been attacked by the Japanese, would it have become immoral for us to join that war, or would it always have been immoral for us to sit it out?

What about the American Revolution? Our people were oppressed to a degree, but nothing like we have seen in modern times. Was it wrong for the States to wage war on the British, or was it a nobel cause?

If terrorists operating under the support of one nation attack another, is the attacked nation justified in moving against the government sponsor?

Is there ever any situation where it becomes immoral to NOT stand up and do something, even if a nation has not personally been attacked, (the Holocaust example)?