dt and jrp: Vito's "no" put Sol in a bind: if he simply accepted it, he would be unable to build his wholesale business the way he envisioned it. I can't see him just walking away from that. I agree that, if Sonny had kept his mouth shut, the other families would not have gone along with a hit on Vito for fear of starting a war. But I believe Sol would have attempted the hit on Vito anyway, and presented the other families with a fait accompli. If he succeeded, there was still a chance that Hagen could convince Sonny to come to terms and avoid a destructive war. If he failed, the Corleones would come after Sol and Tatt, not the rest of them. His fait accompli would put the other families in the position of supporting him to avoid a war--as he told Michael, "The Tattaglias are behind me with all their people. The other families will go along with anything that prevents a war." He was probably right.

The event that precipitated the war was not the attempt on Vito's life per se. It was Michael's killing of Sol and Mac. The novel points out that the NYPD issued a decree: all illegal activity by the Five Families would be shut down until Mac's killer was given up. The Families sent an emissary to the Corleones, requesting that they give up Mac's killer so that business as usual could resume. They were told that the affair did not concern them. That's when the war started.