I should have been clear - I meant a war between the states, analogous to the myriad wars between European countries in the meantime.

Empires and Federations are cool that way - the member states don’t waste resources fighting each other endlessly.

I see. You don’t even need a federation, a customs union usually suffice. E.g. there have been zero wars within the European Union for example. Although I’ll admit, looking at the internal struggle is an unfair simplification as European parties (as with the United States) have plenty of that external violence going on.