Final answer:
Mussolini resigned on July 24, 1943, due to the loss of support following military failures and the Allied invasion. His ousting led to Italy's surrender and change in allegiance during World War II.
Step-by-step explanation:
Benito Mussolini resigned from office when the Grand Fascist Council, with the coalition of former fascist supporters, military officers, liberal politicians, and King Victor Emmanuel III, concluded that Mussolini must be removed from power. This decision took place on July 24, 1943, following a series of events that discredited his leadership, including significant Italian casualties in the Soviet Union, and the Allied victories in North Africa and Sicily. Mussolini's resignation was part of a broader context of World War II, where Italy had initially been an Axis power, but following his ousting, Italy negotiated an armistice with the Allies, leading to the country's surrender on September 8, 1943, and an eventual shift of allegiance in the conflict.