One of the last acts of Don Corleone as Godfather is to bring back the peace by calling for a meeting of the five families. You can watch the scene here.
Corleone makes the case for why narcotics are something he wants to stay out of. Then another guy stands up and makes the case for narcotics. Corleone says he came there to reason and find the peace, and Barzini declares that the narcotics trade will commence despite what Corleone wants, and that Corleone will give up a lot of his power. Corleone gives a monologue where he doesn't seem to say much, and then just like that it's over.
Why did he give in so easily? He didn't even seem to put up a fight or to argue his case. I don't understand why he felt it was necessary to give Barzini and Tattaglia and his other enemies exactly what they wanted just to keep the peace. Surely they all wanted peace as well. Yet it seems like Don Corleone just got slapped in the face at the meeting.