In the midst of the Great Depression, with the memory of the carnage of World War I still relatively fresh and the American economy in shambles, the rise of Hitler had been greeted with relative lack of interest in the United States. Even once war broke out, popular opinion among Americans remained strongly isolationist. Congress accordingly passed a series of laws known as the Neutrality Acts that basically prohibited trade with belligerent nations. These acts were applauded by Republicans and many isolationist groups, including the America First Committee, which counted celebrity pilot Charles Lindbergh among its members and opposed any involvement in European affairs.
Franklin Roosevelt wanted to stay out of the war too, and in fact had refused to sanction Japan for its invasion of China earlier, but as France fell he became increasingly convinced that the United States needed to do something to support the British as they stood alone against Nazi Germany. He first secured passage of the so called "Cash and Carry" amendment to the Neutrality Acts, which allowed the United States to provide military equipment to Great Britain as long as the British paid for goods in full and provided transport for them.
In the midst of the Blitz, Winston Churchill informed the President that "the time is near when we shall no longer be able to pay cash." Roosevelt responded by ushering in the Lend-Lease Act, which he compared to letting a neighbor borrow a garden hose to put out a fire. The Act provided for massive aid to Great Britain (and, before long, the Soviet Union) and aid was sent on transports guarded by US convoys. This move was accompanied by a large propaganda effort that helped to, along with images of Nazi air attacks on British civilians, stir up support.