The involvement of many of the Allies in World War II was natural and inevitable -- they were invaded or under the direct threat of invasion by the Axis.
When Hitler announced that Germany would "liberate" the ethnic Germans in Czechoslovakia, England and France recognized the growing danger of a second world war caused by German expansionism. England and France were politically unprepared for war, however, and appeased Hitler with the Munich Pact -- effectively compelling Czechoslovakia to cede its frontier districts to Germany.
In March 1939, Hitler seized the rest of Czechoslovakia and began to move on Poland. Britain and France guaranteed the integrity of Poland, but Germany and the Soviet Union signed a secret pact in August 1939. With the pact, Stalin bought time to build up his strength at the expense of Britain and France, and Hitler gained a free hand to deal with Poland. When Hitler's army invaded Poland on September 1, 1939, World War II began.
While German forces overran western Poland, Soviet troops entered from the east to claim their portion of that country. France and Britain declared war on Germany and mobilized their forces.
Meanwhile, the United States remained largely isolationist. They viewed the war as a European conflict. After World War I, Americans had little stomach for sending their boys overseas to die.
Congress passed the Neutrality Act of 1937, making it unlawful for the United States to trade with belligerents. But President Roosevelt was strongly in favor of getting the U.S. into the European war and pressed for involvement overseas. He soon convinced Congress to amend the Neutrality Act to permit munitions sales to the French and British. On March 11, 1941, Congress passed the Lend-Lease Act, which gave the President authority to sell, transfer, or lease war goods to the government of any country. This spelled the virtual end of American neutrality.
However, the American people were still against sending their soldiers to fight in a foreign war. The Japanese Empire took care of this by bombing the American naval base at Pearl Harbor, Hawaii, on December 7, 1941.
On December 8, 1941 Congress voted that a state of war existed between the United States and Japan. On December 11, 1941, Germany and Italy declared war on the United States. Congress then immediately recognized the existence of a state of war with Germany and Italy, and also rescinded an article of the Selective Service Act prohibiting the use of American armed forces beyond the Western Hemisphere.
See also: The Axis