FDR brought us through two enormous national crises: the Great Depression and World War II. Was he indispensable? The answers are probably and certainly, in that order, for the following reasons:
1. The Great Depression: Roosevelt called out Hoover for failing to balance the budget in the 1932 campaign. He didn’t have any innovative ideas on economic issues. What he did have was a willingness to try just about anything, because, unlike Hoover, he thought the dangers of inaction to American liberal democracy were greater than the costs of experimenting and failing.
You can break Roosevelt’s actions into three groups. His welfare state legislation was helpful to his contemporaries and obviously has stood the test of time. His Keynesian fiscal and monetary actions were only intermittently successful, as he was not completely committed to them. His corporatist legislation simply didn’t work. Much of it was struck down by the Supreme Court; the country was better off without it.
Was there anyone around who, realistically, could have done better? Certainly, there were no Republicans. I think you have to give him the benefit of the doubt.
2. World War II: Roosevelt was a brilliant war president. He saw the dangers coming and did his best to prepare the country even though the population was predominantly isolationist before Pearl Harbor. He managed to provide Britain and the USSR with a lifeline through Lend-Lease in spite of, not because of, public opinion. He picked excellent military leaders and stuck with them. He succeeded in mobilizing the economy for war to a greater degree than had ever been imagined. The Allies never fell out over war aims, as could easily have happened, while he was president. Finally, most of the international institutions that were created to keep the peace after the war were conceived during his administration.
No one else could have done all of this. FDR passes the indispensability test with flying colors.