Nazi Germany's invasion of Poland on 1 September 1939, marked the beginning of World War II. Despite being an Axis power, Italy remained a non-belligerent until June 1940. The participation of Italy in the Second World War was characterized by a complex framework of ideology, politics, and diplomacy, in which its military history took place often heavily influenced by external factors. The imperial ambitions of the Fascist regime, which aspired to restore a "Roman Empire" in the Mediterranean (their Mare Nostrum), collapsed due to defeats in Greece and East and North Africa. In 1943 Benito Mussolini was ousted and arrested by order of King Victor Emmanuel III, provoking a civil war. The northern half of the country was occupied by Germans and made a collaborationist puppet state (with more than 600,000 soldiers), while the south was governed by monarchist and liberal forces, which fought for the Allied cause in the Italian Co-Belligerent Army (at its height numbering more than 50,000 men), helped by circa 350,000 partisans of disparate political ideologies that operated all over occupied Italy.