I've been consuming the darkness that is wartime histories from the past three or four centuries and I feel like I've encountered a lot of people who had what they believed to be justifiable reasons to launch wars against other powers. There are people who thought they had divine right to a particular position of power and so would launch a war to assert that god-given right. There are people who believed in a citizen's right to have some (any) say in how their tax money gets used in government and so would fight wars over that. People would fight wars to, as John Cleese once said, "Keep China British." Many wars are started to save the honor of a country/nation. Some are started in what is claimed to be self-defense and later turns out to have been a political play instigated to end what has been a political thorn in their sides.
In all this time, I've struggled to really justify many of these wars, but some of that comes with the knowledge of what other wars have cost in terms of human carnage and suffering. For some societies in some periods, the military is one of the few vehicles to social mobility (and I think tend to think social mobility is grease that keeps a society functioning). Often these conflicts come down to one man's penis and the inability to swallow their pride to find a workable solution unless at the end of a bayonet. These conflicts also come with the winning powers taking the opportunity to rid themselves of political threats and exacting new harms on the defeated powers (which comes back around again the next time people see each other in a conflict).
So help keep me from embracing a totally pacifistic approach to war. When is a war justifiable? When it is not only morally acceptable but a moral imperative to go to war? Please point to examples throughout history where these situations have happened, if you can (though if you're prepared to admit that there has been no justifiable war that you're aware of, I suppose that's fine if bitter).