Ok, the posts are too many to read all of them, so I apologize if this was said before, and if it was not said then I am AMAZED no one thought of it:

No one broke the promise that Vito made, not even Michael. the "Peace" was broken by the attempt to kill Mike after his father's funeral. To plot and begin to execute an assassination is breaking the peace, to fail in carrying it out does not mean that they (the other families) did not try, and trying is breaking the peace.

Now we know that Mike and Vito predicted it would happen, but no act was made to break the peace from the Corleone's side, what Mike did was REACT to a real threat; best defense is offense, simple.

As for Tom, it was a tactical move... in times of turbulence, no leader wants to be second-guessed; Tom was older and with greater experience working alongside Vito than Mike had, Mike needed absolute power, and he moved Tom out to protect him from becoming a liability to the family, he was an asset as a lawyer. in GF2 that lead to Tom temporarily taking over the family after the Lake Taho assassination attempt.