On April 9, 1940, Germany simultaneously invaded Norway and occupied Denmark, and the war began in earnest. On May 10, German forces swept through Belgium and the Netherlands in what became known as "blitzkrieg," or lightning war. Three days later, Hitler's troops crossed the Meuse River and struck French forces at Sedan, located at the northern end of the Maginot Line, an elaborate chain of fortifications constructed after World War I and considered an impenetrable defensive barrier. In fact, the Germans broke through the line with their tanks and planes and continued to the rear, rendering it useless. The British Expeditionary Force (BEF) was evacuated by sea from Dunkirk in late May, while in the south French forces mounted a doomed resistance. With France on the verge of collapse, Benito Mussolini of Italy put his Pact of Steel with Hitler into action, and Italy declared war against France and Britain on June 10.
On June 14, German forces entered Paris; a new government formed by Marshal Philippe Petain (France's hero of World War I) requested an armistice two nights later. France was subsequently divided into two zones, one under German military occupation and the other under Petain's government, installed at Vichy. Hitler now turned his attention to Britain, which had the defensive advantage of being separated from the Continent by the English Channel. To pave the way for an amphibious invasion (dubbed Operation Sea Lion), German planes bombed Britain extensively throughout the summer of 1940, including night raids on London and other industrial centers that caused heavy civilian casualties and damage. The Royal Air Force (RAF) eventually defeated the Luftwaffe (German Air Force) in the Battle of Britain, and Hitler postponed his plans to invade. With Britain's defensive resources pushed to the limit, Prime Minister Winston Churchill began receiving crucial aid from the U.S. under the Lend-Lease Act, passed by Congress in early 1941.