The mafia used the war as a money-making opportunity, but made every effort to not run afoul of the Feds, whom had every ability to wipe them out as enemy agents if they even gave a whiff of helping the Axis because of their Italian heritage. In other words, they kept a low profile if they knew what's bests for them. The various families had more than enough business to go around with the war, and again kept the inter-family fights to a minimum so to not attract attention that would needlessly foul up a good thing.

However, certain kingpins were tapped to use their connections to weed out potential spies that were working the docks whose workers were members of unions that were controlled by the mafia. Also, their connections back in Sicily were tapped to gain important and valuable intelligence about the strength and position of Axis troops on the island at the time of the invasion by the Allies.

Of course this was not done because of some desire to be part of the war effort. There were favors traded for this.