I looked up a relevant passage in the novel yesterday. When Tessio is arguing about Barzini's incursions, and wants to recruit more men, Vito replied: "I gave my word, and I can't break it," or something to that effect. Tess replies that Barzini's actions are "provocation enough," presumably to constitute "breaking the peace."
Basically correct TB. Tessio is correct also. But Vito's words are only meant to buy Michael sufficient time to implement his plan without any interference from Corleone subordinates. As such, Vito's words are intended to placate Tessio.
