As a follow on to the posts from Bob's World and The Curmudgeon ...

Whenever "State" and "Nation" aren't perfectly congruent and homogeneous, you have the seeds for violence. That is as true today, e.g. East Africa and Tutsi vs. Hutu, as it was a couple millennia ago, e.g. Ionian Greeks vs. Attic Greeks.

A second observation is that the weaker side will always fight "dirty," since they believe (rightly or wrongly) the rules get made by the stronger side and are always written to favor his interest.