Final answer:
FDR called the German U-boats that operated in the North Atlantic as 'rattlesnakes,' due to their threat to the Allied shipping in the Atlantic during WWII.
Step-by-step explanation:
Franklin D. Roosevelt (FDR) referred to the German U-boats that operated in the North Atlantic during WWII as rattlesnakes. These submarines posed a significant threat to Allied shipping, particularly in relation to the Lend-Lease program, which was essential for supplying Britain and the Soviet Union with food, materials, and munitions. The U-boats, part of Germany's strategy to maintain a blockade and starve the UK into submission, targeted not just military but also merchant vessels. Confrontations increased in 1941, with several U.S. destroyers being torpedoed which resulted in American casualties and intensified the naval conflict even before the U.S. formally declared war. The U-boats constituted a considerable part of the German unrestricted submarine warfare campaign, prompting the U.S. to shoot them on sight, and causing outrage due to their attacks without warning, which was against international law.