Is Sonny had not made the gaffe, Sol would have not had everything lined up the way he liked, but he still had the financial protection of Tatt and Barzini. There was also some kind of legal cover because Sol had McCluskey in his pocket. Perhaps the legal cover was not as great as it would have been with Vito's support, but it would have to do until Sol could chip away at the Corleone territories.

I think the war began with the shooting of Vito. After it was clear he wass not mortally wounded there was a second attempt on his life, which Michael thwarted at the hospital. For that, Michael got his jaw punched out by McCluskey, and in turn Sonny killed Bruno Tattaglia. The war was on.

Sol was not strong enough to present Vito's death as a fait accompli to Barzini. Had he done so, Barzini and Tattaglia would have had to choose between a full scale war, or cutting their losses by offering Sol up as a sacrificial lamb to the Corleones, and just waiting for the right opening.

As Sol well know, blood is a big expense.