/lit/ - Literature

I'm looking for a particular kind of story. Basically, one in which the opposing sides start out as close as possible. Best friends, brothers, squad members - whatever, not important. What's important is that a difference in ideology drives them apart. Mind you, I am NOT talking about personal differences that would drive apart a couple or family members. I mean they come before an important, but unexpected situation, and both have opposing views about what must be done and compromise is impossible. Think of the Jedi/Sith, Chaos/Imperium, etc. They're almost the same but diverge so completely on one issue that they cant' ever come together again.

So the formula would be AxB are inseparable --> AxB come across X --> AxB have opposing views about how to deal with X --> A and B separate as enemies and tragedy ensues.

The tragedy aspect is important. I don't care what they disagree about on, nor if that ever gets resolved. I just want characters doing unthinkable harm to people they love because they think they're right. Except both sides think that, and it escalates into nothing but empty suffering driven by opposing ideologies.

I know this is very specific, but maybe I've missed something that has this. And yes I've already read Horus Heresy, since it fits perfectly with the criteria.