In the aftermath of WWI (or as it was known then, the Great War) and especially after the stock market crash of 1929 and the ensuing Great Depression, the American public was not interested in getting involved in foreign wars again. Even as Hitler came to power, American popular opinion and a majority of Congress were strongly non-interventionist.

There were four Acts, passed in 1935, '36, '37, and '39, with the first two banning arms and "materiel," or war supplies, to "belligerent" nations, and the second two allowing an exception for nations that could pay cash up front and ship the supplies themselves (mostly Britain and France).

The Neutrality Acts complicated the social and political landscape of the U.S. for FDR, who was stuck between the hard place of American isolationism (with its lingering cultural trauma of WWI) and the rock (read: crashing boulder) that was the threat of the Axis powers.

Most significantly, the Neutrality Acts created a legal roadblock for much of the preventative measures that FDR wanted to take in order to protect the American people from enemy military aggression. They existed as a legal and historical precedent for those who opposed his preemptive measures, making it very easy for his detractors to conjure arguments against any sort of U.S. involvement in the war. All they had to do was point to the laws.

This is why FDR had to be so crafty when it came to implementing the programs of the Lend-Lease Act. He was maneuvering within a framework that was, in a way, working against him and his best intentions for the country.