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. 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. 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. 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.

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