I have read an FDR biography and it goes into great details about FDR's difficulties in convincing a largely isolationist population, a reluctant military and an almost hostile Dept of State to support the Allies in WWII. It did not go into why. Was his motivation moral, personal or geopolitical? I know he was a political pragmatist but this was one issue that did not look like a vote winner before Pearl Harbor.
P.S. Call me cynical but I do not think any politician's public speeches are a good indication of what he/she really thinks. I consider private conversations and letters or even the opinions of people who know them well to be much better sources.