If you read only one book on the morality of war, this should be the one. Walzer is the preeminent modern Just War Theorist, and this is still the definitive text on the subject - even if you don't agree with its entirety. I certainly don't.
If you are an "absolute pacifist," you have to answer why it would be morally justifiable to stand and watch the unmitigated horrors of genocide that have gone on throughout history without end other than force of war, and Walzer gives many examples here. Walzer, respectfully, challenges Ghandi stating that an army would not be repelled by its own sense of horror in mowing over millions of innocents, if that sense is absent or has been removed by the collective reprogramming of the state. Unfortunately, the Holocaust, was neither the first, nor the last such case in point, and Walzer uses Cambodia as a more recent example. If you realize this and say you are a "contingent pacifist," like Einstein and Russell, who would admit that "some circumstances call for war," well, then, you may as well claim to be a Just War Theorist. That way you can call upon a long, evolutionary history of what defines just and unjust causes for war and its prosecution. Walzer details that history and its rationality brilliantly.
If you are a so-called "realist," who makes a point of saying there is no such thing as an "unjust" cause for war, as long as you reasonably decide that it is in your own best interest, you are entitled to your opinion. However, you have to admit that anyone else could claim the same, no matter how ridiculous it may sound - and therein lies the rub: most people actually do call some reasons "ridiculous." We live in a world that actually behaves as if it has a desire for being moral, not one that lives as if it doesn't! Modern Just War Theory, including Walzer, admits that there may not be easy answers to what determines "normative" behavior on an international scale, but we do have to start somewhere, and do so through the spirit of cooperation and consensus that has been built up over centuries. If we don't, we all may as well go back to living in caves and cry "it's every man for himself."