What megan said. (ETA - And what tommyrot said too, esp. about the ass-saving. The focus is on what we did when we got in the war, not how long it took us to show up.) There was a strong isolationist movement in the US. We had our own troubles. Per the wikipedia entry on President Franklin Roosevelt:
Foreign policy, 1937–1941
The rise to power of dictator Adolf Hitler in Germany aroused fears of a new world war. In 1935, at the time of Italy's invasion of Ethiopia, Congress passed the Neutrality Act, applying a mandatory ban on the shipment of arms from the U.S. to any combatant nation. Roosevelt opposed the act on the grounds that it penalized the victims of aggression such as Ethiopia, and that it restricted his right as President to assist friendly countries, but public support was overwhelming so he signed it. In 1937, Congress passed an even more stringent act, but when the Sino-Japanese War broke out in 1937, public opinion favored China, and Roosevelt found various ways to assist that nation.[40]
In October 1937, he gave the Quarantine Speech aiming to contain aggressor nations. He proposed that warmongering states be treated as a public health menace and be "quarantined."Meanwhile he secretly stepped up a program to build long range submarines that could blockade Japan. When World War II broke out in 1939, Roosevelt rejected the Wilsonian neutrality stance and sought ways to assist Britain and France militarily. He began a regular secret correspondence with Winston Churchill discussing ways of supporting Britain.
For foreign policy advice, Roosevelt turned to Harry Hopkins, who became his chief wartime advisor. They sought innovative ways to help Britain, whose financial resources were exhausted by the end of 1940. Congress, where isolationist sentiment was in retreat, passed the Lend-Lease Act in March 1941, allowing the U.S. to give Britain, Russia, China and others $50 billion of military supplies 1941–45. In sharp contrast to the loans of World War I, there would be no repayment after the war. Roosevelt was a lifelong free trader and anti-imperialist, and ending European colonialism was one of his objectives. Roosevelt forged a close personal relationship with Churchill, who became Prime Minister of the UK in May 1940.
In May 1940, a stunning German blitzkrieg overran Denmark, Norway, the Netherlands, Belgium, Luxembourg, and France, leaving Britain vulnerable to invasion. Roosevelt, who was determined to defend Britain, took advantage of the rapid shifts of public opinion. A consensus was clear that military spending had to be dramatically expanded. There was no consensus on how much the U.S. should risk war in helping Britain. FDR appointed two interventionist Republican leaders, Henry L. Stimson and Frank Knox, as Secretaries of War and the Navy respectively. The fall of Paris shocked American opinion, and isolationist sentiment declined. Both parties gave support to his plans to rapidly build up the American military, but the isolationists warned that Roosevelt would get the nation into an unnecessary war with Germany. He successfully urged Congress to enact the first peacetime draft in United States history in 1940 (it was renewed in 1941 by one vote in Congress). Roosevelt was supported by the Committee to Defend America by Aiding the Allies, and opposed by the America First Committee.
Roosevelt used his personal charisma to build support for intervention. America should be the "Arsenal of Democracy," he told his fireside audience. In August, Roosevelt openly defied the Neutrality Acts by passing the Destroyers for Bases Agreement, which gave 50 American destroyers to Britain in exchange for base rights in the British Caribbean islands. This was a precursor of the March 1941 Lend-Lease agreement which began to direct massive military and economic aid to Britain, the Republic of China and the Soviet Union.