Guys... what did Tessio actually do to betray?

He arranged the meeting between Mike and Barzini at which Mike would be murdered.

oh... ok, maybe I need to revisit the film, but what meeting is this?

The meeting never actually took place. Tessio approached him with it during the funeral. Vito correctly advised Mike that whoever comes to Mike as a mediator between Barzini and the Corleones is the the traitor.

When Tessio comes to the compound to pick up Mike for the meeting, this is when Tessio is killed.

oh yes... i do remember this... this is the last conversation Michael has with Vito in the garden. BUT... how does Vito know this??? As for Barzini to meet w/ Michael, there has to be SOMEONE whome is notified with this information. Why must the traitor have been the messenger?

Just like your question re: "It was Barzini all along", it all comes down to Vito's instincts and his ability to read between the lines.

Vito just knew that Barzini would never want to mediate (Barzini wants to control everything) and therefore, any meeting offered that suggests otherwise is made under false pretenses and is a trap. To set up his trap, Barzini needed an ally and therefore, the messenger is the traitor.

well... Vito's intelligence and insight (but not clairvoyance) being understood... could Barzini not, just as well, have requested that he meet w/ Michael by some one completely untainted???

The traitor Vito referred to was not a function of being tainted or not. It was a function of behavior: the one who set up the meeting.

Vito in his old age was not really sharp. He was slipping, as Solozzo points out. There is a key sentence from tessio where he accepts his betrayal by saying "tell michael it was strictly business". But for that statement, I would have assumed that vito's prophecy was not to be taken too seriously.

In a word, "No".

The meeting would have had to have been set up by someone Mike trusted and someone who was close to him. The novel gives a much better explanation of this. According to the novel, soon after Vito died Tom Hagen asks Mike how "they'll come at him". Mike replies that Vito instructed him it would be through someone close to him. Mike further adds that Barzini would not approach Tom because he was Irish (and according to Barzini that would make him untrustable) and they wouldn't approach Neri because he used to be a cop (also untrustable). Rocco Lampone wasn't close enough so that left Tessio, Clemenza or Carlo.

So, Barzini approached Tessio and gave him a promise that Tessio would allowed to be a boss in his own right after Michael was killed. Tessio figured that Mike couldn't win in a war against Barzini so he took that offer and arranged for a meeting in Brooklyn (his turf) where Michael "would be safe".

Obviously Tessio and Barzini both underestimated Mike.

Good point SC, but I don't think Barzini would have used Carlo. I think it came down to Tessio or Clemenza because by that time Barzini had to know that Mike suspected Carlo of setting up Tom. He also had to have known that Carlo was a relative lightweight who would not have been able to give Mike the assurances he needed. That is would have to be either Tessio or Clemenza is emphasized in the film where Tom says "I always thought it would be Clemenza," and Mike replies, "Its the smart move. Tessio was always smarter.")

I don't think so either. It was Puzo who wrote that statement (that Mike thought the final three choices were Tessio, Clemenza and Carlo).

Well you can't argue with Mario

ok... well, maybe i was not really clear with what i meant... I understand that it would have to be someone close to Mike, to present this to Mike, but why would this person have to be a traitor??? ie. let's just say that they came to Clemenza with the proposition... all Clemenza would have to do is tell Mike that the meeting would be held at his turf. the only way that I see that it may have to be a traitor is if Barzini just felt the need to affirm his own safety. Clemenza would not have had to have known this, in this example (as with Tessio as well). Anyway, The whole point of it being some one close to Mike to present it is for this very reason... so that he does not suspect there to be something malicious going on. Am I correct in my assessment?

Frank, I think what was going on was the manner in which Barzini would want a meeting with Michael to "straighten everything out."
Michael was going to move the family to Vegas and supposedly Tessio and Clemenza were going to able to start their own family (which Clemenza eventually did and which Pentangeli took over in GFII). We know there was bad blood between Barzini's people and Tessio and Clemenza's people because they say so in the "You're out, Tom" scene, during which Michael assures them everything will be worked out. In fact Michael had no intention of working everything out, because if he did, he could have called a commission meeting same as Don Vito did when he swore he would never the the one to break the peace. Barzini could also have done that if he really wanted to keep the peace, which he did not. Barzini knew that if given the time, Michael could retrench in Las Vegas, Clemenza could hold New York, and Michael would regain his strength to become a threat in the future (what he didnt see coming was Michael's bold move to assassinate all of them on the same day). Accordingly since Barzini didn't want a meeting with Michael that was on the up and up, he tried to set Michael up with a phony olive branch. He wanted Michael to think that he was coming to him personally and he wanted Michael to have the assurance he would be safe. Vito and Michael both knew that in order to assassinate Michael on Corleone controlled territory he would need someone close to Michael to betray him. Knowing this he probably put out feelers to Tessio and to Clemenza, and Tessio took the bait. Tessio saw the Corleone position as getting weaker, and he did not see that Michael's move to Vegas would ultimately strengthen the family not weaken it further. I also do not think he had the faith in Michael that he had in Vito. So, being a business man ( Led to his final ride Tessio says, tell Mikey it was not personal....I always liked him.) Tessio made what he thought was a smart business decision. He made the deal with Barzini that he could keep his territory if he arranged for a meeting where Michael would be assassinated.

Bottom line, both Barzini and Tessio underestimated Michael and it cost them their lives.

Thank YOU... THAT'S IT! That's what i was getting at... i was missing the whole "if Barzini wanted peace, he would arrange a meeting w/ the 5 families" angle. Thus, if he came to Mike w/ a meeting (which Vito knew he would), it would be phony.