The Five Families War was a major conflict that saw the fall of the Barzini, Tattaglia, Cuneo and Stracci Families.
The War began after the death of Virgil Sollozzo and police officer Mark McCluskey by Michael Corleone. Sollozzo had previously arranged a hit on MIchael's father Vito Corleone, which had not succeeded, but left the Corleone Family hungry for revenge.
As soon as the war started the Five Families went straight to the matresses, hiding out in various hotels and venues. Immediately, the Corleone Family began to experience difficulties, not only because they were outnumbered, but because the enemies money was not out in the open using shylocks and bookmakers, rather, it was hidden in the docks and behind numerous fronts.
However, the skill of Sonny Corleone ensured that they managed to score several decisive strokes against their enemies, including a major distribution racket run by Artie Manzanero and the Cuneo Family, yet secretly backed by the Barzinis in Midtown. At this time, many of the rival Families hubs and warehouses were taken, such as the Stracci Rail Hub. He then set up a system of surveillance, observing the heads of the other Families, but they soon ducked into hiding.
Believing that the other Families were no longer a threat, the security loosened up, yet this was seen to be a front, when Sonny was lured out and murdered by members of the other Families. The recovering Don Vito called a hasty peace at a sitdown, and the war was seen to be over.
However, the Barzini Family continued to chip away at the Corleone's territory, much to the chagrin of Corleone caporegime Salvatore Tessio, who was forbidden to act, and instead made a deal with Barzini that would result in the demise of Michael Corleone, who had become the new Don.
Tessio's plot failed, and he was executed, as were the heads of the rival Families in a decisive move that wiped out all opposition to Michael's rule. The new heads of the Families quickly sued for peace.