One war? Am I miscounting? There is the Spanish American war, the Mexican war, the war of 1812, the US Civil war (slave war), there were numerous Indian wars during the manifest destiny. Several wars in the Caribbean and Central America.

I’m sorry if this seems pedantic since it doesn’t affect your point, but I feel like it deserves some clarification.

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.

I agree with your comment and also recommend the linked pages, but I think you have to count the American revolutionary war as a war, too.

Though like so many it was not resolved on the battlefield but through lobbying and bribery in parliament. Unlike, say, the Viet Nam war in the USA, the American skirmishes had a negligible impact on the British purse.