It seems that regions with many small independent states often suffer from frequent wars (e.g., the Mesopotamian city-states, the Greek city-states before the Macedonian conquest, the Arab tribes before Muhammad, the Italian city-states during the Renaissance). Can anyone name some counterexamples?
On the other hand, it seems that the unification of a warlike nation frequently leads, not to an end to war, but to expansionist wars (as opposed to internal ones). After the Macedonians had conquered and unified Greece, they went on to try to conquer the whole world. After Muhammad had unified the Arabs, they aggressed against other peoples. In my opinion, a bellicose nation should rather fight among themselves than harass other nations.