The Aventine Secession was the withdrawal of the Italian Socialist Party from the Italian Chamber of Deputies in 1924-25, following the murder of Giacomo Matteotti. It was named after the Aventine Secession in ancient Rome, and heralded the assumption of total power by Benito Mussolini and his National Fascist Party and the establishment of a one-party dictatorship.
In 1923 the Acerbo Law replaced proportional representation. It meant that the largest party, providing it had at least 25% of the vote, gained 2/3 of the parliamentary seats. Following the elections in April 1924, Mussolini obtained 2/3 of the votes anyway giving him a clear majority under either system. After his outspoken accusation of the Fascist Party's underhand methods of political corruption and voter intimidation, the Socialist leader Giacomo Matteotti was abducted and murdered by Amerigo Dumini and various other thugs whose names were linked to the Fascist Party. There is considerable dispute among Historians as to the level (if any) of Mussolini's knowledge and involvement.
The resulting uproar after the Matteotti murder left Mussolini vulnerable, having been forced to dismiss numerous members of his entourage, including General De Bono, Chief of Police and Head of the MVSN. In late July 1924, the Socialists began a boycott of parliament, with the aim of forcing the King to dismiss Mussolini.
Without the socialists the vote of no confidence in Mussolini was a failure. King Victor Emmanuel III was disinclined to invoke further violence from the Fascist squads, and thus allowed Mussolini to keep his position as Prime Minister. Mussolini declared that the Socialists had forfeited their seats and the party was banned in early 1925. In January 1925 Mussolini declared a dictatorship and started a series of repressive measures designed to destroy opposition.
The Secession served only to aid Mussolini in his consolidation of power as it eliminated all meaningful parliamentary opposition, and deprived the King of any excuse to dismiss him.