There is a line of thought which indicates that Roosevelt clearly understood that the primary way to end the economic crisis of the 1930s would be through the mass industrialization and production caused by the United States' entry into war. Yet, at the same time, the horrors of World War I, the uncertainty about where Europe was heading, as well as the inward focus of the United States citizens caused by the economic depression of the time prevented a full scale embrace of war. Roosevelt understood the need to slowly move policy and perception towards the efforts of war in Europe. The attack on Pearl Harbor allowed the complete galvanizing of US support that was needed to go to war. However, before that Roosevelt was slowly moving towards war by supplying troops and weapons to England and the Allied powers in their war against the Axis powers. Additionally, Roosevelt slowly moved America to war by recognizing Communist Russia. Understanding the land hungry drive of Germany, in particular, Roosevelt knew that when the time came to confront Hitler in befriending the Russians, he would have an ally.
Great answer above. An additional thing FDR did to take us slowly out of isolationism was to sell weapons to Britain as part of the "Cash and Carry" program - whereby the British paid cash for American weapons, and then carried them on British ships.
In this way, America could plead that it was merely business, not an alliance, and the fact they were carried on British ships meant American ships would not be sunk by U-Boats, dragging us into war as it did in WWI. Great idea, although Britain soon ran out of cash and ships, so FDR replaced that program with Lend Lease and the Destroyer Deal mentioned above.
After WWI, and especially in the 1930s, the American public was very opposed to the idea of involvement in foreign affairs. This was manifested in the Neutrality Acts that were passed during that decade.
After Hitler came to power and started to grab more and more of Europe, Franklin Roosevelt became concerned about the Nazis and wanted the US to do more to prevent their rise. This was especially true after WWII started with the invasion of Poland.
During this time, FDR tried to move the US towards greater involvement. The major things he did during this time were:
- The Destroyers for Bases deal
- The Lend-Lease Act
- Using US warships to escort convoys as far as Iceland
- The Atlantic Charter
All of these moved the US away from isolationism and towards involvement in WWII.