I think the peak was reached in the 50s, the slow decline began in 1963 when the witness of Valachi, Hoover could no longer say that the Mafia didn't exist, also in the 60s and 70s the different ethnic groups that had remained under the heel of the Italians,like the blacks and the Hispanics started to come out from the ghettos and to compete with the Mafia in the 70s the Irish with Danny Greene in cleveland and Bulger in Boston create serious damage and almost defeated the Mafia.