In the book, of course, the final massacre is more limited than in the movie. I think one of the reasons is hinted at in the chapter telling the narrative of Vito's life. It says that, by the late 30's, Vito had achieved an agreement among all the major crime organizations in the country. That chapter says, "The agreement covered only spheres of influence and an agreement to enforce peace in the underworld." (emphasis added)

The day after McCluskey was killed, the NYPD put out word that all deals were off until the killer was turned over: "Massive raids began all over the city." The Five Families turned to the Corleones, but were told the matter was none of their business. That is the point where the families joined together in open warfare against the Corleones.

It's always seemed to me that the war had at least two phases. Those would be the Tataglia-Barzini campaign over drugs, and then the Five Families War over the McCluskey killing.

I've always had a sense that after McCluskey was villified and the police reestablished relations, Cuneo and Stracci pulled back in the war, if not dropped it completely. That sense, though, may come entirely from the fact that they weren't held accountable for Sonny.

Assuming that killing a police captain would be considered a breach of the peace by all parties, it seems to me that Stracci and Cuneo were not just allowed but required to make efforts to bring the killer to "justice," that the Corleones would see their efforts as consistent with the system that Vito had designed, and that that is why Stracci and Cuneo survived the book.

Thoughts appreciated.

Excellent analysis!

That is certainly a viable theory.

One of the reasons people have offered for Cuneo and Stracci not surviving the film is simply directorial license.

Puzo wrote something along the line of Stracci's Family being the least powerful but most well disposed to the Corleones of the Five Families in opposition to the Corleone Family.

As you point out the killing of a police captain would have been a serious breach of formal and informal rules and so Stracci and Cuneo would have had little choice but to go along with the war.

But since Stracci and Cuneo weren't the heavy hitters or instigators of the war, Vito and Michael may well have decided it wasn't necessary to take them out. They'd fall in line once Barzini and Tattaglia were removed. Vito may have reasoned, "Why make unnecessary enemies". Also since Cuneo's true power and business was still unknown to the outside world a statesman like Vito might not have seen much value to the Corleones or the larger organized crime structure of "outing" Cuneo by murdering him. As Sollozzo said "Blood is a big expense".

The movie amped up the violence to emphasize that Michael was settling the score for once and for all with anyone who had opposed his Family-no matter what....

Very fine analysis, MP!
I agree that the primary reason for all being massacred in the film (including moving back Moe Green's assassination) was for FFC to make a more dramatic, violent denouement to the movie. It's worth keeping in mind that the ultra-vengeful, hyper-controlling, never-give-a-foe-a-pass Michael was a creature of GFII, not of the novel. So, it'd be logical for him (in the novel) to conclude (as others here said) that "blood is a big expense." And anyway, the murder of Barz and Tatt was a shot fired across Cuneo's and Stracci's bows. They were no real threat to him, or to Clemenza, his successor.

Lilo, I think it's right that if the plan was developed with an eye toward keeping the "expense" to a minimum, then it would probably be Vito's counsel that led to that decision.

TB, I think Michael in the novel is actually closer to GF2 than he is to GF1. One of the most curious changes from the book is when movie Michael says, "It's not personal, Sonny. It's strictly business." In the book he says, "It's all personal, every bit of business....If a bolt of lightning hit a friend of his, the old man would take it personal....Accidents don't happen to people who take accidents as a personal insult."

In the book, Michael says that the massacre is partly vengeance for Sonny and Appolonia. I don't think the book's Michael would let Stracci and Cuneo off for tactical reasons.

That's it in a nutshell.