During the final stages of the Olive Oil War, Emilio Barzini began to see a change in the wind as his boss, Giuseppe Mariposa, began to lose power to Vito Corleone, once a small time neighbourhood padrone and now leader of the increasingly powerful Corleone family. After all attempts to kill Corleone failed, and Mariposa killed Barzini's friend "Tits" purely on suspicion, Emilio sold Mariposa out to Corleone. Barzini inherited the remnants of the Mariposa organization and preceded as boss of his own family. He also gained a seat on the newly formed Commission.
Barzini's master plan
"I didn't know until this day that it was... Barzini all along."
In 1945, Barzini saw a new chance to achieve to supplant the Corleone family and gain dominance in New York, plotting with Virgil Sollozzo and Philip Tattaglia, the latter of whom he used as an unwitting scapegoat. Barzini almost succeeded, pushing Vito Corleone to call for peace after the death of his eldest son, Sonny. From the late 1940s to the mid 1950s the Barzini family was at the height of its power and became the strongest family in New York. They began moving in on Corleone territories, which forced Salvatore Tessio, Vito's most savvy capo, to made a secret deal with Barzini.
Future of the Family
However, the wily Don Vito discovered Barzini's plot and his son Michael took revenge, having Al Neri kill Barzini, who was succeeded by Paul Fortunato. Under Fortunato's leadership the family expanded its narcotics operation and focussed on Manhattan's garment sector. Fortunato resented the Corleone family's hypocrisy by getting into the drug business, but defended Michael Corleone against Carlo Tramonti's accusations at a Commission meeting. After this relations softened.
In The Godfather: The Game, the Barzinis are the most powerful of the families that Aldo Trapani fights. The Barzinis play the role as the main antagonists of the video game, being the last family to be wiped out in 1955 and Emilio Barzini was shot by Trapani at the Foley Square Courthouse. They can kill the player with a few shots, having the most powerful buttonmen.