What policies did FDR adopt during 1939-1941 to protect American interests?
Franklin Roosevelt attempted to walk a thin line between involvement in the War and aiding the allies. He succeeded for a time, but ultimately, the U.S. was drawn into the war, at least partially because of Roosevelt's policies.
The British Navy had long served to protect America from attack from the Atlantic; but at this point the British navy was stretched thin. At the request of Winston Churchill, Roosevelt agreed to sell 50 "overaged" American destroyers to Britain in exchange for 99-year leases on military bases in the British Empire. He claimed it was for the protection of the Western Hemisphere, saying that his action was
the most important action in the reinforcement of our national defense that has been taken since the Louisiana Purchase.
At the same time, in June, 1940, Roosevelt authorized the creation of the National Defense Research Committee, which secretly studied the possibility of creating an Atomic Bomb. This after he received a letter from Albert Einstein suggesting that the Germans might succeed in doing so first.
The "lend lease" program was a means of aiding Britain but also a means of circumventing the Johnson Debt Default Act, which prohibited foreign aid to any country which had defaulted on its World War I debts to the U.S. Britain had done so, and Roosevelt used the lend lease program as a way to get around the prohibition. Senator Burton Wheeler opposed the lend lease program and in a sarcastic reference to the old AAA program commented:
The Lend Lease program is the New Deal’s triple A foreign policy; it will plow under every fourth American boy.
Finally, in August, 1941, Roosevelt met with Churchill off the coast of Newfoundland and they jointly issued the Atlantic Charter, which called for freedom of the seas, self determination for all people, economic cooperation and equal access to raw materials. A strange agreement for a President who insisted that he and his country were neutral.
In the time between the start of WWII and the US entry into that war, Franklin Roosevelt believed that it was important for the US to support the Allies as much as possible. He felt that the US should actually participate in the war, but knew that the American public would not support US entry into the war. Therefore, he took a number of steps to (as he thought) protect US interests.
Basically, FDR took steps to help the Allies. He instituted such policies as the Lend-Lease Act by which the US essentially gave weapons and other supplies to the Allies. He ordered the US Navy to help to escort convoys carrying supplies from the US to Britain. By doing these things, he was taking steps that were tantamount to entering the war on the side of the Allies. Germany tolerated these actions because they did not want all-out US participation in the war.
Overall, then, FDR adopted the policy of helping the Allies as much as he could because he believed an Allied victory was in the interests of the US.