Unless the Romulans were complete idiots, and I'm not saying they aren't, though I realize they're coloured by delusions of superiority, they would know the Federation is a lesser evil than the Dominion, especially after the attack on the Homeworld. The fate the Founders had for Cardassia would be shared by the Romulans. And how on earth would they fight the Dominion if the Dominion got practically half the galaxy?
The Romulans' hesitation is more evidence for my hypothesis that, objectively, the Dominion had been decisively beaten after the Prophets closed the wormhole to their military traffic. The Romulans, not being idiots, would have joined the war earlier if the Federation and Klingons had been faltering significantly. Only subjectively, from the eyes of Sisko, Ross, and other people who might die in it, does the Dominion War look like a more closely contested engagement. This is much like World War II, at the time, seemed a lot more nearly run than, in hindsight, we know it was.
As for who started the war, I wonder how this will fly

: Vreenak was right. Sisko started the war. He committed an act of war by blockading the Dominion's lines of communications to its own territory. I'm not questioning the correctness of this decision, but rather its diplomatic effect.
Alternate firestarters:
The captain of the Odyssey--violated Dominion space with a Federation battleship.
Sisko again--violated Dominion space in a cloaked Federation warship.
Tain, Obsidian Order, and elements of the Tal Shiar--tried to commit a counterpower strike against the Dominion, throwing a little genocide into the bargain.
Sisko again--violated Dominion space with a Federation warship during the same attack on the Founders' homeworld, destroyed Dominion vessels in the process.
Gowron--unchecked aggression against Cardassia, Dominion subject. Possibly instigated by Changeling Martok however, but Gowron didn't need much help.
Assume for a moment the Dominion isn't automatically evil, and it's hard to see how they couldn't have considered the Alpha Quadrant an existential threat. They were invaded by every major power, except the Klingons, and first.