Final answer:
Mussolini was expelled from the Socialist Party for his support of Italy's involvement in WWI, which contradicted the party's pacifism. He shifted from socialism to militarist nationalism and founded the Fascist Party, becoming Italy's dictator in the 1920s.
Step-by-step explanation:
Benito Mussolini was expelled from the Socialist Party due to his strong support for Italy's involvement in World War I, which was in stark contrast to the party's stance on pacifism. Mussolini believed the war would act as a catalyst for a socialist revolution but this view conflicted with the prevailing socialist thought, which typically opposed nationalist wars.
After his expulsion, Mussolini renounced socialism and embraced a militarist nationalism, leading to the formation of the Fascist Party. This transformative period in Mussolini's political life marked a significant shift from his initial revolutionary socialist ideology to an autocratic leadership style that characterized his rise to power in Italy.