Oh, sure, B-of-B, I guess so...if you want to call that 'breaking the peace'.

Personally I never thought of it that way.

By Barzini breaking the peace I was only referring to him having Michael killed; the only thing which Vito made clear he would not stand for. So still, the betrayal of Tessio to set up Michael for assasination would not have taken place until after Vito's death.

If you want to get technical though, I suppose you're correct that Barzini, by '..sending his people to take over Corleone Family territory...' would be considered breaking the peace.

A move which apparently Vito and Michael expected or at least were not disturbed by...hence their calm reaction and orders to Tessio and Clemenza to take no acton until after the move to Nevada.
They knew someone within their Family would turn traitor...they only had to wait to find out who it would be.

Apple