The enslavement of human beings was evil, period. I have an idealistic view that evil (in the form of institutional slavery, genocide or whatever) must be stopped, and the cost might seem very high but is worth it.
The cost of war is not just very high, it's always too high, and therefore not cost effective. The peace dividend is also very high, and can be used to stop war dead in it's track.
Pacifism is the way. War is the problem. Peace is the result of impeccable logic. War is the result of specious logic. As simple as all this is, most people just can't seem to get it. I don't understand why, but I know it is because people are not logical enough. Why people choose the dark side over the light is a mystery, and doesn't bode well for us as a species.
Ah yes, I'm sure that pacifism would have been quite effective at stopping things like slavery and Nazism!