<P> 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 27 May 1941 by sinking the German battleship Bismarck . </P> <P> In November 1939, the United States, who were taking measures to assist China and the Western Allies, 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 . </P> <P> In December 1940 Roosevelt accused Hitler of planning world conquest and ruled out any negotiations as useless, calling for the US to become an "arsenal of democracy" and promoted Lend - Lease programmes of aid to support the British war effort . The US started strategic planning to prepare for a full scale offensive against Germany . </P> <P> 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 would make major contributions to the Axis war against the USSR; in Romania's case partially to recapture territory ceded to the USSR . </P>

Who started world war 2 and why did it start