Complex sentence: World War II began on 3 September 1939 and on 10 June 1940, Italy declared war on the Allies and by September had invaded France, British Somaliland and Egypt; preparations had also begun to occupy Greece.
Split into simpler sentences: World War II began on 3 September 1939 and on 10 June 1940, Italy declared war on the Allies. By September 1940, the Italians had invaded France, British Somaliland and Egypt; preparations had also begun to occupy Greece.