i just finished this book called "Mr. S: My Life with Frank Sinatra" by George Jacobs, who was Sinatra's valet for 15 years. amongst many other succulent and sordid tidbits, i read about how Sinatra reclined with mobsters, entertaining them at his legendary all-night parties and dealing with them in matters of business. politicians also frequented Sinatra's many galas, including none other than JFK himself, a dapper, witty, sex-crazed man who thrived off of the lifestyle that the Man with the Golden Voice and his buddies could offer him. JFK made a deal with the mob that if they helped him win the presidential election, he wouldnt oppress them when he assumed the role of Chief Executive. the mob graciously assisted him, but soon after his victory, Bobby Kennedy convinced JFK that being associated with such shady characters as Frank Sinatra and Sam Giancana wasn't the best PR move, and so JFK summarily went back on his word. he allowed Bobby to doggedly pursue venerable godfathers and their cohorts from Las Vegas to New York, all under JFK's name. and so basically, the mob decided that this wasn't good for business, and so they conspired amongst eachother, and had Oswald do the deed. an interesting side note can be added here; Richard Nixon was tight with the Mafia and had many links that ran deep into several preeminent and prodigious crime families. he also held less than congenial feelings for the Kennedys, and so he was gleefully aware of the mob's plot. they then had Jack Ruby kill Oswald, to keep their clandestine workings under the radar. Giancana, then the unquestioned Don of Chicago, openly boasted of the successful assassination. this, among other things, led Congress to call him into their presence in 1975, right before which Giancana was executed before he could talk. several other gangsters involved in the organized murder admitted decades later, on their deathbeds, to what has become the most romanticized killing in the 20th century.
here's the site where i got the details on a lot of this stuff:
then on sinatra's well-documented mob ties: