Civil war, and Revolution 'may' be justifiable depending on how strongly people feel about their own personal liberties and how they feel the ruling power is handling them. But one man's revolutionary is another man's terrorist so it is all very subjective.

As far as going to war with another nation I feel there could be several reasons to justify it:

1) A preemptive strike against a warlord/dictatorship that is about to acquire nuclear weapons, or other types of weapons of mass destruction that would threaten the peace and security of the world. This was the US's excuse to invade Iraq which proved to be false.

2) In defence of another nation or ally under threat or attack.

3) To institute a regime change if the occupying regime is considered a hostile threat to its own people or surrounding territories and in order to promote future stability.

It still depends on the exact situation and details. War should be a last resort in any case when diplomacy fails but sometimes it can be necessary. In the future though I think war will become largely obsolete. It already has to some extent as regards to other countries in possession of nuclear weapons. You can bully around third world dictators and warmongers but once a country acquires nukes war becomes a war of politics and trade agreements.