In a prelude to the Castellammarese War in New York City, New York mob boss Joe Masseria attempts to mediate a dispute between Chicago gangsters Al Capone and Joe Aiello However, Aiello ignores Masseria's peacemaking and later threatens him for interfering. Masseria then goes to Aiello's main supporter, Detroit mobster Gaspar Milazzo, to get his support in ending the Chicago gang war. However, Milazzo refuses to help and is supported by a faction of the Brooklyn Castellamarese organization, led by Cola Schiro, and Buffalo mobster Stefano Magaddino's organization. Their actions precipitate a general split among American mafiosi and convince Masseria that these mobsters are plotting against him.
New York gangster Charles Luciano, representing the "Young Turks" faction (which included mobsters such as Vito Genovese and Frank Costello), and Ciro Terranova begins secretly negotiating with Salvatore Maranzano to persuade him to declare independence from Joe Masseria.
Brooklyn mobster Joe Parrino is killed by members of the Castellamarese organization. Following his death, control of the Brooklyn Castellamarese is turned over to Sicilian boss Salvatore Maranzano by Buffalo, New York mob boss Stefano Magaddino. Chicago and New York police detectives establish a connection between the weapons used in the St. Valentine's Day Massacre and the murder of New York mobster Frankie Yale, implicating Chicago boss Al Capone in Yale's death.
The US Federal Bureau of Narcotics replaces the Narcotics Division of the Prohibition Unit.
Murder, Inc. gunman George Defeo is killed in New York by the Shapiro Brothers.
Benjamin Bennett, a New York mobster brought to Chicago by George "Bugs" Moran as muscle for the North Side Gang, disappears shortly after his arrival.
January - Brooklyn mobster Cola Schiro disappears after being forced to pay $10,000 to Joe Masseria. After Schiro's disappearance, Joe Parrino succeeds Schiro as leader of the Brooklyn Castellamarese faction.
January 5 - Ciro Terranova issues a press statement regarding the dramatic robbery at the Judge Albert Vitale Fundraiser dinner in New York on December 1929. Terranova claims that he is being used as a scapegoat for the split in the New YorkTammany Hall political organization. He also claims to be a "man of peace" and asks to be left alone.
February 4 - Chicago gangster Julius Rosenheim is killed shortly after agreeing to become a police informant.
February 26 - Bronx mobster Gaetano Reina is killed from a shotgun blast by mobster Vito Genovese while leaving his aunt's home. Reina is later replaced by Joseph Pinzolo Reina, an ally of Masseria, had been suspected of secretly negotiating to defect to Maranzano's organization before his death.
March 14 - The New York State Apellate Court orders the removal of Magistrate Albert Vitale due to his ties to organized crime figures such as Ciro Terranova and Arnold Rothstein and an unexplainable $10,000 deposit to his bank account.
March 17 - Released from prison in Philadelphia, Al Capone returns to Chicago to resume his war against mobster Joe Aiello.
April 13 - Clinton G. Price, district attorney of Juneau County, Wisconsin and political figure within Milwaukee, is severely wounded by a shotgun blast at his home. Price dies the following morning.
May 31 - Gaspar Milazzo, founder of the Detroit crime family, is shot and killed (although other sources claim he was stabbed to death) while walking through a Detroit fish market.
June 9 - Chicago Tribune journalist Alfred Lingle is shot and killed in a train station in Chicago, Illinois. The Chicago newspapers would promise a $55,000 reward for information on the murderer. It would be rumored that Lingle had extensive connections to organized crime.
June 30 - Thomas Somneiro, a Chicago Outfit lieutenant, is found dead in Chicago's West Side. He was strangled to death.
July 5 - Cleveland, Ohio mobster Joe Porello is killed, along with his bodyguard, by an unidentified gunman while dining at a restaurant owned by mobster Frank Milano. Milano would succeed Porello as leader of the Cleveland syndicate.
July 31 - Thomas McNichols and James "Bozo" Schupe, two smalltime bootleggers, shoot each other to death on Madison Street in Chicago.
August 15 - Peter Morello, consiglierie to Joe Masseria, and his bodyguard Giuseppe Piranio are killed by gunmen employed by rival boss Salvatore Maranzano. The gunmen possibly include the mysterious mobster known only as Buster from Chicago.
September - Forming a secret alliance with the remnants of deceased mobster Tom Reina's organization, Salvatore Marazano arranges to have Joe Pinzolo, the new Reina family boss, killed in early September. Pinzolo would be killed by Bobby Doyle outside the offices of the Lucchese crime family. The hit is reportedly planned by Lucchese bosses Tommy Lucchese and Gaetano Gagliano.
September 23 - Al Capone rival and Unione Siciliane President Joseph Aiello is killed in a drive-by shooting outside a friend's home. Aiello's death eliminates a major threat to the Chicago Outfit. Agostino Loverdo is placed by Capone as head of the Union Siciliane, however the organizations value greatly declines over the decade, quietly dissoving by the late 1930s.
November 5 - Mobsters Steve Ferrigno and Alfred Mineo, allies of Giuseppe Masseria, are killed by Vincinzino Sperlli,Vito Genovese and Joe Valachi, Maranzano's gunman outside Ferrigno's home in New York. Masseria narrowly avoids the ambush himself.
Anthony Parino future boss of St. Louis Family is born.