Christianity and war

I know there are at least two people who read this blog who actually know stuff about Christianity, and I'm wondering if you (or other people! people I don't know!) could help me with something I find confusing. I've read the New Testament, and I know a little about the early history of the Christian church, but I know more or less nothing about more recent theology. Jesus is very clear in the New Testament, mostly. "Love your enemies, do good to them which hate you, bless them that curse you, and pray for them which despitefully use you. And unto him that smiteth thee on the one cheek offer also the other; and him that taketh away thy cloak forbid not to take thy coat also." (from Luke, and also from Matthew.) He hangs out with lepers and prostitutes and tax collectors, and I don't remember a single instance in which he does anything violent (though I'm ready to be corrected on this). There's some talk of how God will judge people, but really nothing about how anyone should carry out that judgment now, and in fact various injunctions against judging, more or less on the concept that it's God's job to do that and we're not God. At least that's how I remember it, from my New Testament class seven years ago.

So obviously Tiller's murder raises some questions for me, but so do the Crusades and pretty much every war that any Christian organization has blessed or waged. What's up with that? How do Christian theologians justify war and retribution? There's plenty of bloodthirsty behavior in the Old Testament, so I can see where Jews are getting it. I know basically nothing about Muslim theology, but it does seem like there's not the same crazy pacifism in Muslim tradition. I mean, it seems like it's mostly a situation of "people will be people, no matter what the scriptures actually say," but I do wonder how the people who claim to buy into that scripture then square it with their own desires for violence and retribution. Especially if there are people who grapple with it in a serious and honest way.