I think that "the peace" existed before Sollozzo entered the picture: respect one another's territory, do each other services if you can. I think that the peace conference restores that broader peace, with the change that the Corleones will protect the drug trade.
Apparently, Barzini broke the peace first. However, as the novel states, even Tom knew that Vito's agreement was not true to his character. He knew that a day of reckoning was in the future.
"Generosity. That was my first mistake." "Experience must be our only guide; reason may mislead us." "Instagram is Twitter for people who can't read."
