Complex sentence: Italy was an important member of the Axis powers in World War II, until it switched sides to the Allies in September 1943 after ousting Mussolini and shutting down the Fascist party in areas (south of Rome) controlled by the Allied invaders.
Split into simpler sentences: Italy was an important member of the Axis powers in World War II, until 1943. It switched sides to the Allies after ousting Mussolini and shutting down the Fascist party in areas (south of Rome) controlled by the Allied invaders.