Why did Michael Corleone go through the trouble of assassinating Stracci and Cuneo, too, instead of limiting the bloodletting to just Barzini and Tattaglia?
Actually, in the novel, Michael did only hit Barzini and Tattaglia.
According to the original source material, Stracci and Cuneo were considered less antagonistic of the Five Families, and they never put their full weight behind Barzini and Tattaglia.
Stracci, too, was old-fashioned and never dealt in prostitution, but because his business was on the waterfront it was impossible for him not to be involved in the drug-smuggling traffic. Of the five New York Families opposing the Corleones his was the least powerful but the most well disposed…
Cuneo was one of those men who loved children and carried a pocket full of sweets in the hopes of being able to pleasure one of his many grandchildren or the small offspring of his associates. … He was one of the few Dons who had never been arrested and whose true activities had never even been suspected. So much so that he had served on civic committees and had been voted as “Businessman of the Year for the State of New York” by the Chamber of Commerce.
The Godfather — A Novel by Mario Puzo
In the book, Stracci sided against the Corleones, but apparently without physical opposition, while Cuneo had public persona to maintain, so any publicity related to the Five Families War would have been damaging.
But none of that was the case in the film. We know that in the movie, Michael did take out all four heads of the fellow Five Families members, and the reason he had to was, Michael had killed a New York City Police Captain, breaking a long-standing mafia tenet, which the other family heads could not condone, nor appear to support in any way, even if only by way of indifference.
So the other families felt they needed to separate themselves from the Corleones, and that included Stracci and Cuneo.
Sonny had already raised some bad blood by going to the mattresses anyway, so along with Michael’s hit on Sollozzo and McCluskey, that was enough for Barzini to influence the remaining of the Five Families to stand against the Corleones in the Five Families War that lasted three years.
“How bad will it be?” (Michael)
“Probably all the other families will line up against us. But, it’s alright. These things have to happen once every ten years or so…gets rid of the bad blood. You gotta stop ’em at the beginning. Like they shoulda stopped Hitler at Munich, they shoulda never let him get away with that, they were just asking for big trouble…” (Peter Clemenza, after he shows Michael the gun to be used to assassinate Sollozzo and McCluskey)
The Godfather (1972)
Clemenza understood what was to come for the Corleones for what Michael was about to do. There was no way any of the other families would look the other way.
By the time Vito Corleone called for a meeting of the Five Families to negotiate a truce, it was four families against one.
It was a testament to just how powerful the Corleone Family was — and Sonny, in particular, as the Corleone field general — that they were able to take on the other four families and still limit their losses enough that they could still call for a meeting to end the bloodshed.
But, after Vito’s death, the remaining heads all had motivation to come after Michael. He had to act quickly to cut the heads off the hydra that was the Five Families of New York City, and that meant the end for Cuneo and Stracci, as well as Barzini and Tattaglia.
In the end, Michael was a new don and he also needed to build his own reputation and a new legacy for the Corleones. Part of that meant cleaning out the old guard, so that he could negotiate with other up-and-coming mafia leaders who would be more open to establishing peaceful and profitable terms with the Corleones, rather than simply trying to muscle them out, as the older, more experienced dons would have likely tried to do.
– Peter Ramirez