FDR actually did a little more than that. Before the war erupted and Ike was appointed he was a busy guy. In September of 1940 he started the draft. He pushed and signed the Lend Lease act in March of 1941. He launched the prewar industrial buildup. For example, we broke ground on the famous Willow Run Bomber plant 6 months before Pearl Harbor. We were in a defacto war with Germany in the Atlantic for months before Pearl Harbor. When presented with the concept, he was fully behind the Manhattan Project.
And during the war, probably most important of all he made sure the Brits were second among equals. Ultimately he made sure we were the lead partner and he always diplomatically reminded the Brits of that in several small ways.
That mentality ensured we didn’t get an idiotic repeat of the British leadership in WWI.
Lots of things not to like about Roosevelt in the Depression, and in his handling of the Russians. But it’s a bit much to say all he did was appoint Ike. That’s just not historically accurate.