OK, to take of from your point, Sollozzo wanted to rule out any possibility for a revenge from the Corleone family so that he could materialize his business aspirations without any risks to his life or operations. And the threat of a full-scale war was just a mean to achieve the deal in the negotiations since nobody really wanted that. A mean he would have preferred to avoid himself as well.

But in this case, why would the war have started in the first place if Sonny said no for the deal and Mike wouldn't have killed McCluskey?

btw: Ristiisa means Godfather in Estonian.