A U-boat raid up the Mississippi is treated as an operational counterfactual. Reaching the Gulf would require several Type XIV "milk cow" supply submarines to refuel an attack boat across the Atlantic and station another tanker off the American coast. The attack submarine would need another refueling after leaving the river and further support for the return passage. Flood season might help inland access, but the long chain would make the sortie exceptionally risky and unlikely to return.
Notable Point · 2:46:28-2:47:58
What a U-boat Mississippi raid would demand
A speculative Mississippi raid exposes the long refueling chain a German attack submarine would need to reach, fight, and return.