I don't think nationalities so much prevent war as give some reason for war to take place. Possibly the most annoying thing about the civ 2 AI was that it attacked over meaningless patience and codiality factors. Also, the only wars ever fought (other than skilled human defensive stuff) were all out attacks on the opponents cities.
Nationalities can help in both respects: The AI (and humans) should be encouraged to liberate members of their nationality, just as real nations do. Wars would be fought for a reason (subjegated peoples), and the former boundaries of an empire would gain some importance.
Secondly, empires would, as has been pointed out, be discouraged from wars meant to conquer foreign lands. These two point balence each other out: more logical wars, but fewer wars of conquest.
If your strategy is Howitzer rushing, this change is not for you. But if you can use a variety of tactics, this will only make the game more believably real.
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