Although the United States did not officially enter the war until 1941, Roosevelt did everything possible to support Allied powers through several initiatives. With the memory of WWI still fresh in the collective national memory, the American public was war-weary and suspicious of any attempt to join Allied efforts.
The 1935 Neutrality Act forbade any American exports of arms or ammunition to belligerent nations, and the Johnson Act of 1934 prohibited loans to any country who had not repaid any debts incurred during WWI. Undeterred, Roosevelt repeatedly lobbied Congress to repeal the Neutrality Act; his efforts paid off when Congress passed the Pittman Bill in November 1939, which allowed the United States to sell arms to nations on a cash-and-carry basis.
The American military also had reservations about helping England. Military leaders hypothesized that England would soon surrender after the French collapse, and the weapons would end up in the hands of the Germans. So, Roosevelt had to come up with a solution which would satisfy the American public, the American military, and the dictates of the law. With Churchill warning of English defeat if more help was not forthcoming, Roosevelt and Congress came up with the 'Destroyers For Bases' agreement on September 2nd, 1940.
The agreement would exchange 50 obsolete destroyers for 99-year US Air Force and Navy land leases in the Caribbean and Newfoundland. The English felt this was a lop-sided deal, but Churchill knew his back was against the wall. He accepted the deal without further argument. Roosevelt, the master tactician, managed to make lend-lease agreements with at least 30 nations who were pushing back against Nazi aggression. With almost $50 billion in financial assistance to these countries, Roosevelt managed to hold America back from involvement in the war until it was ready. SO, Roosevelt succeeded in maintaining the appearance of neutrality while maneuvering American resources to defeat the Germans.
By the time America entered the war after the Pearl Harbor attack, America was excellently positioned to leverage its strong position with allies to defeat Germany. Please refer to the links below to read about Roosevelt's war-time strategies.