One thing I've wondered is:

Michael, under the guise of meeting for a "peace meeting" (where he'd be assassinated), had the Bosses of the four other families murdered, in essence leaving Michael temporarily the only living Boss of the Five Families. But, wouldn't there have been some fall out from this? Consider that the Five Families engaged a long, bitter and bloody war after Vito was shot.

Wouldn't at least one of the new Bosses of the other Families have retaliated, re-launching the War? Or at least some Capos or Underbosses loyal to Barzini, Tattaglia, Cuneo and Stracci launching campaigns against the Corleones in reponse?

How did the Corleone Family manage to maintain any peace? I would think Michael's action would've just relit the War officially.

The only answers I can think of are that Michael became in essence "Boss of Bosses", with the new heads of the other Four Families becoming in essence puppets of him, or, that Vito/Michael had made secret, long term arrangements with powerful and power hungry Underbosses or Capos in the other four families, who were sympathetic to the Corleones, that he'd whack their current Boss and in turn they'd become Boss--Which is how I could see someone like Don Altobello coming to power. Or that the strike impressed so many soldiers that they move to the Corleone Family.

We do know that Vito and Michael were planning the Massacre for several years, which included Rocco Lampone slowly building a secret regime. It's possible that many of Rocco's soldiers were culled from the other four families, and the way the hits went down to me implies that the Corleone Family had informants in the other four families who were willing to give up their Bosses--Telling Michael where the other Bosses would be so his buttons would know when and where to be.