Largely diplomaticly and financially. The US actively supported secessionists with guns and money. Being fair the little federation was never very stable, but the US got involved immediately to ensure it never found it’s footing.

Imagine that, if during the first Constitutional Convention in the late 1780’s, if Europe had sent envoys to separate factions with an assload of weapons and whispered “hey, you really gonna let them get away with that shit? I got your back bro if this gets bloody…”

That’s kinda what we did.