France kept its fleet, which the United Kingdom attacked on 3July in an attempt to prevent its seizure by Germany. The air Battle of Britain began in early July with Luftwaffe attacks on shipping and harbours. The United Kingdom rejected Hitler's peace offer, and the German air superiority campaign started in August but failed to defeat RAF Fighter Command, forcing the indefinite postponement of the proposed German invasion of Britain. The German strategic bombing offensive intensified with night attacks on London and other cities in the Blitz, but failed to significantly disrupt the British war effort and largely ended in May 1941. Using newly captured French ports, the German Navy enjoyed success against an over-extended Royal Navy, using U-boats against British shipping in the Atlantic. The British Home Fleet scored a significant victory on 27May 1941 by sinking the German battleship Bismarck. In November 1939, the United States was taking measures to assist China and the Western Allies and amended the Neutrality Act to allow "cash and carry" purchases by the Allies. In 1940, following the German capture of Paris, the size of the United States Navy was significantly increased. In September the United States further agreed to a trade of American destroyers for British bases. Still, a large majority of the American public continued to oppose any direct military intervention in the conflict well into 1941. In December 1940 Roosevelt accused Hitler of planning world conquest and ruled out any negotiations as useless, calling for the United States to become an "arsenal of democracy" and promoting Lend-Lease programmes of aid to support the British war effort. The United States started strategic planning to prepare for a full-scale offensive against Germany. At the end of September 1940, the Tripartite Pact formally united Japan, Italy, and Germany as the Axis powers. The Tripartite Pact stipulated that any country, with the exception of the Soviet Union, which attacked any Axis Power would be forced to go to war against all three. The Axis expanded in November 1940 when Hungary, Slovakia, and Romania joined. Romania and Hungary later made major contributions to the Axis war against the Soviet Union, in Romania's case partially to recapture territory ceded to the Soviet Union. Mediterranean (1940–1941) In early June 1940, the Italian Regia Aeronautica attacked and besieged Malta, a British possession. From late summer to early autumn, Italy conquered British Somaliland and made an incursion into