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.
Another example of what you are talking about is the Mongols’ outward expansion after Genghis Khan united the various clans in the late 12th and early 13th centuries. The Hanseatic League in the Baltic region was a federation that made war on other states in the late medieval period.
If you’re looking for a counterexample, the Holy Roman Empire is not a great one, but on the whole I’d say that the component parts of it were not really at each others’ throats during much of the Middle Ages.