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Not exactly gangsters but pirates played a large role in the battle for New Orleans during the War of 1812.
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yes. WWII. Italian mob helped with Italian campaign.
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Given that the answers so far are not sufficient because they are only subsuming the Wikipedia link pointed out by Clint Eastwood , I line out the answer given in Wikipedia and give out the sources in case of link rot.
Yes, Mafia gangsters were very helpful in the planning of Operation Husky, the invasion of Silicy which is for many gangsters their isle of origin.
Charles "Lucky" Lucianoβs associates contacted Sicilians to draw harbor maps and provide photos of the coastline for the Naval Intelligence.
Vito Genovese, another Mafia boss, offered his services to the U.S. Army and became an interpreter and advisor to the U.S. Army military government in Naples.
Sicilian underworld personalities and friendly Sicilian natives were reported and contacted during the Sicilian campaign.
Luconi, Stefano. "Italian Americans and the Invasion of Sicily in World War II." Italian Americana 25.1 (2007): 5-22.
McCoy, Alfred W. The Politics of Heroin in Southeast Asia. New York: Harper and Row, 1972.
Newark, Tim. Mafia Allies: The True Story of America's Secret Alliance with the Mob in World War II. Saint Paul: Zenith Press, 2007.
Raab, Selwyn. Five Families: The Rise, Decline, and Resurgence of America's Mast Powerful Mafia Empires. New York: Thomas Dunne Books, 2005.
I do not count Lucianos help for preventing strikes because he only helped because the government agreed to commute its sentence.