After the Commission meeting, Vito tells Tom: "Until this day, I didn't know it was Barzini all along." Perhaps he saw the same hints that we saw at the meeting: Barzini sits at the head of the table, leads the discussion, jumps to the concluding statement ("Then it is agreed: The traffic in drugs will be permitted, and Don Corleone will give it protection in the East"). BUT:

How could Vito not have known, way before the meeting, that it was Barzini all along? In explaining to Tom how he came to that revelation, he says, "Tattaglia is a pimp...alone he could never have outfought Santino." Well, Tattaglia was a pimp way before the war began--didn't Vito know that then? And, at the beginning of the Commission meeting, he says, "I want to thank Don Barzini for helping me to arrange this meeting." Since the beef was between Vito and Tattaglia, why would Vito reach out to Barzini, if not to acknowledge that he was the key belligerent? Maybe Vito was slippin'...?

I thought there might be some direction in the novel about this in the, but I can't find my copy. However, maybe Vito's words (as put into this mouth by Puzo and FFC) was a little inaccurate. Maybe they should have had him say that his suspicions regarding Barzini were not confirmed until today. I think that in the novel Vito states to Tom that Barzini never committed himself. So, maybe that's an indication of Vito lack of certainty until the meeting.

Oli, I looked in the novel just now. There was no "it was Barzini all along" anywhere. In his narrative describing the Dons at the Commission meeting, Puzo writes: "The closest ally to the Tattaglia Family was Don Emilio Barzini." A page later, describing Tattaglia, Puzo writes: "In a curious way his almost victorious war against the Corleone Family had not won him [Tatt] the respect it deserved. They [the Dons] knew his strength had come first from Sollozzo and then from the Barzini Family."

I always took that to mean that Vito knew that someone was backing Tattaglia but, until the meeting, he did not know who it was. (BTW, I don't think Vito says "alone" when talking about Tatt outfighting Santino, TB, but the sentiment is certainly there.)

I agree that Barzini seemed to be the most likely suspect, but Vito probably wanted to make sure of it before he took any action. After all, his primary motivation at the meeting was to ensure Michael's safe return.

Tom has the closest thing to the line in the book. After the peace conference, Vito tells Tom that Barzini never committed himself and doesn't get caught on the losing side, and Tom replies, "You mean he was behind Sollozzo and Tattaglia all the time?". Then Vito calls Tattaglia a pimp, and says it's enough to know Barzini had a hand in it.

The whole course of the war is murky in the book. Barzini's not committed, Barzini's supporting Tattaglia, all five families (seemingly including Barzini) are actively at war. Which is it, when? It's hard to make sense of the ending if all the families were actively fighting the Corleones, but it seems they were. I assume that the explanation is that Puzo was writing quickly.

Pete, in the novel law enforcement starts cracking down on all of the families' activities as a result of the McClusky murder and they demanded that the Corleones give up the murderers. Eventually, Sonny realizes that the Corleones are overmatched.

What's interesting to me about that is that the war apparently was costing so many lives that the Families' counter-blows declined and then eventually ceased. Sonny was jubilant and said he would pour it on. That was the root of the deception by the families; make Sonny think he was winning, get him to be careless, and then strike. So, what if Sonny takes Tom's advice and ceases his attacks on the families and even sues for peace? I also question what the families were trying to accomplish by deceiving Sonny? Was it purely to end the killing by killing Sonny? Certainly, they could have accomplished that by suing for peace. For me, that's where the murkiness comes in.

My take, for what it is worth!

Vito certainly suspected there was more to Sollozzo’s proposal than what Sollozzo was telling Vito – why Vito sent Luca to find out what ‘they had under their finger nails”

However I believe Vito didn’t suspect someone was backing Tattaglia until his murder attempt that the hit on the top, most powerful, Numero Uno Mafia Don would not have been carried out on just the say-so of Tattaglia, a pimp

Hence that someone must be Barzini “the most likely suspect” as evident by Vito reaching out to Barzini "I want to thank Don Barzini for helping me to arrange this meeting."

The meeting confirmed Barzini was that someone

Sure thing Woltz, Vito’s “primary motivation at the meeting was to ensure Michael's safe return” so that Vito could / would get Michael to re-establish the Corleones' glory, standing, reputation etc. to do all the dirty work, thus leaving a murderous legacy for “I never wanted this for you” son Thanks! Pop

No doubt Vito was slippin'

I always assumed that the "I want to thank Don Barzini.." line was Vito subtly indicating that he knew Barzini was involved because Barzini was quick to help arrange a meeting once the Corleones had "lost". But it never did quite make sense that he wouldn't have naturally assumed Barzini from the start given that the Tattaglias were considered a weak family (they were the weakest IIRC) so would have been stupid to start an all-out war with the Corleones unless they had some allies to actually give them aid (as opposed to just siding with them to avoid a war the way Cuneo and Stracci did in the novel).

I get the meeting confirming that it was Barzini, but Barzini should have been the top suspect from the onset of the war. Sollozzo wouldn't have struck without someone backing him, which was Tattaglia as far as manpower, but with how overall weak the Tattaglia family was it should have been obvious they needed someone strong to back THEM up, and the only person capable and ambitious enough to do that was Barzini.