At the outbreak of World War I in 1914, the United States remained neutral. Most Americans sympathized with the British and French, although many opposed intervention. Initially neutral in World War I, the U.S. declared war on Germany in 1917, and funded the Allied victory. The nation refused to follow President Wilson's leadership and never joined the League of Nations. The United States, effectively neutral during World War II's early stages after Nazi Germany's invasion of Poland in September 1939, began supplying materiel to the Allies in March 1941 through the Lend-Lease program. After the Japanese attack on Pearl Harbor in December 1941, the U.S. was prompted to enter World War II alongside the Allies and helped defeat Nazi Germany in Europe. The United States, having developed the first newly-invented nuclear weapons, used them on the two Japanese cities. Japan surrendered on September 2, ending the war. Participation in the war spurred capital investment and industrial capacity. Among the major combatants, the United States was the only nation to become richer—indeed, far richer—instead of poorer because of the war.