If all it took was the assassination of that one senator, the Rommies (or at least a powerful faction) were just itching to go against the Dominion (the destruction of the prize Tal Shiar fleet would not have gone over that well). Said senator probably was the opposition to joining the war - once he was gone, it was a forgone conclusion.


Your "principles" won't mean a thing when the peoples of the Federation are killed or enslaved. A government's first responsibility is self-defense. They owed the Romulans nothing. The Romulans would have been fine with watching the UFP and the Klingons be destroyed while fighting for the freedom of the Alpha Quadrant.