Sal Tessio

I've become newly obsessed with the GF movies since they've been running alot on AMC and A&E. But I never seem to have completely uninterrupted time to watch them so I keep missing certain scenes. Can someone tell me what exactly Sal Tessio does to betray Michael? I keep seeing Sal being led off to his supposed death. But what did he do to Michael? Thanks alot.

Well, he was suppose to provide safety at a future meeting that Michael was suppose to have. The meeting was with Don Barzini, but Tessio was going to join Barzini and allow Michael to be assasinated at the meeting.