On September 1, 1939, Germany invaded Poland, followed by Britain and France declaring war on Germany on September 3. World War II had begun in Europe. With little fighting between Britain, France and Germany until the following year, this period from September 1939 through April 1940 was called the “Phoney War”.
In 1940, Germany invaded Denmark and Norway beginning in April, followed by a Blitzkrieg of Belgium, Holland and France beginning on May 10, 1940. Western Europe would be occupied until Allied forces arrived at Normandy on June 6, 1944.
With Western Europe occupied, Hitler turned his focus to Great Britain in Operation Sea Lion. Luftwaffe attacks on the Royal Air Force commenced in the summer and fall of 1940. During the Blitz, beginning on September 7, 1940, Hitler turned his focus to attacking British cities. Though the devastation suffered by the British was massive, German attacks neither hampered the British war economy, nor adequately prepared the way for a German invasion. When Germany turned its attention to its eastern front in Operation Barbarossa, the invasion of the Soviet Union commencing in June 1941, the focus on Britain was eased.