Why, with possible exception of Barzini, were the Dons without protection at the time they were murdered?
They were slippin'. They all underestimated Michael.
Who really was a threat to them at that point? The Corleones appeared weak and powerless, and the other Don's, at that point, were already in bed with Barzini.
Don Cardi
Five - ten years from now, they're gonna wish there was American Cosa Nostra. Five - ten years from now, they're gonna miss John Gotti.