The King declared war on Nazi Germany in September 1939, after the German invasion of Poland. During the quiet period of "phoney war", the British sent to France the most highly mechanized army in the world; together with France they had more tanks than Germany, but fewer warplanes. The smashing German victory in Spring 1940 was due entirely to "superior combat doctrine. Realistic training, imaginative battlefield leadership, and unparalleled initiative from generals down to sergeants." The British with the thinnest of margins rescued its main army from Dunkirk (as well as many French soldiers), leaving all their equipment and war supplies behind. Winston Churchill came to power, promising to fight the Germans to the very end. The Germans threatened an invasion—which the Royal Navy was prepared to repel. First the Germans tried to achieve air supremacy but were defeated by the Royal Air Force in the Battle of Britain in late summer 1940. Japan declared war in December 1941, and quickly seized Hong Kong, Malaya, Singapore, and Burma, and threatened Australia and India. Britain formed an alliance with the Soviet Union (starting in 1941) and very close ties to the United States (starting in 1940). The war was very expensive. It was paid for by high taxes, by selling off assets, and by accepting large amounts of Lend Lease from the U.S. and Canada. The US gave $30 billion in munitions; Canada also gave aid. (The American and Canadian aid did not have to be repaid, but there were also American loans that were repaid.)