FDR was limited in his ability to respond to Britain's call for help in 1940 due to the Neutrality Acts passed by Congress in the 1930's.
The Neutrality Acts consisted of three different laws, but all of them were based around the idea of avoiding foreign wars at all costs. The following are just some conditions of this acts passed by Congress:
1) The US government was forbidden to sell arms to countries involved in war.
2) Forbid American ships from entering war zones.
3) Barred Americans from traveling on ships that would be considered hostile.
4) Prevented American commercial ships from being armed.
All of these conditions show America's dedication to staying out of foreign affairs during this time period.