During this period, Hitler built up his forces on Germany's western frontier. In April 1940, German forces invaded Denmark and Norway. In May 1940, Hitler's forces attacked France, conquering the Luxembourg, Netherlands and Belgium in the process. France surrendered on 22 June 1940. These victories persuaded Benito Mussolini of Italy to join the war on Hitler's side on 10 June 1940.
Britain, whose forces evacuated France by sea from Dunkirk, continued to fight alongside other British dominions in the Battle of the Atlantic. After having his overtures for peace rejected by the British, now led by Winston Churchill, Hitler ordered bombing raids on the United Kingdom. The Battle of Britain was Hitler's prelude to a planned invasion. The attacks began by pounding Royal Air Force airbases and radar stations protecting South-East England. However, the Luftwaffe failed to defeat the Royal Air Force. On 27 September 1940, the Tripartite Treaty was signed in Berlin by Saburo Kurusu of Imperial Japan, Hitler, and Ciano. The purpose of the Tripartite treaty, which was directed against an unnamed power that was clearly meant to be the United States was to deter the Americans from supporting the British. It was later expanded to include Hungary, Romania and Bulgaria. They were collectively known as the Axis Powers. By the end of October 1940, air superiority for the invasion Operation Sealion could not be assured, and Hitler ordered the bombing of British cities, including London, Plymouth, and Coventry, mostly at night.