Why did Mussolini involve Italy
in World War II on the side of
the Axis Powers?

The participation of Italy in the Second World War was characterized by a complex framework of ideology, politics, and diplomacy, while its military actions were often heavily influenced by external factors. Italy joined the war as one of the Axis Powers in 1940, as the French Third Republic surrendered, with a plan to concentrate Italian forces on a major offensive against the British Empire in Africa and the Middle East, known as the "parallel war", while expecting the collapse of British forces in the European theatre. The Italians bombed Mandatory Palestine, invaded Egypt and occupied British Somaliland with initial success. However, German and Japanese actions in 1941 led to the entry of the Soviet Union and United States, respectively, into the war, thus ruining the Italian plan of forcing Britain to
agree to a negotiated peace settlement

The Italian dictator Benito Mussolini was aware that Italy (whose resources were reduced by successful pre-WWII military interventions in Spain, Ethiopia and Albania) was not ready for a long conflict. He opted to remain in the war as the imperial ambitions of the Fascist regime, which aspired to restore the Roman Empire in the Mediterranean (the Mare Nostrum), were partially met by late 1942. By this point, Italian influence extended throughout the Mediterranean. Libya had been pacified under the fascists and was undergoing Italian settlement. A friendly fascist regime had been installed in Spain, and a puppet regime installed in Croatia following the German-Italian Invasion of Yugoslavia. Albania, Ljubljana, coastal Dalmatia, and Montenegro had been directly annexed by the Italian state. Most of Greece had been occupied by Italy following the Greco-Italian War and Battle of Greece, as had the French territories of Corsica and Tunisia following Vichy France's collapse and occupation by German forces. Italo-German forces had also achieved victories against insurgents in Yugoslavia, and had occupied parts of British-held Egypt on their push to El-Alamein after their victory at Gazala.

However, Italy's conquests were always heavily contested, both by various insurgencies (most prominently the Greek resistance and Yugoslav partisans) and Allied military forces, which waged the Battle of the Mediterranean throughout and beyond Italy's participation. Ultimately the Italian empire collapsed after disastrous defeats in the Eastern European and North African campaigns. In July 1943, following the Allied invasion of Sicily, Mussolini was arrested by order of King Victor Emmanuel III, provoking a civil war. Italy's military outside of the Italian peninsula collapsed, its occupied and annexed territories falling under German control. Italy capitulated to the Allies on 3 September 1943.