He sent soldiers to resupply the fort. The South wouldn't let them and eventually took control of it.
Isolationism.
The Great Depression and the fact that the US kinda promised to not do another World War like the Great War caused them to not want to join WWII at first. Then Pearl Harbor happened and they decided to be done with their policy of isolationism.
If you don't have options then there could really be a variety of options. The Japanese bombing of pearl harbor, the atomic bomb, the lend-lease act, the destroyers for bases deal with Britain, and much more.
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