Final answer:
President Roosevelt referred to German U-boats and their crews as "the rattlesnakes of the Atlantic" during the undeclared naval war in the Atlantic before the U.S. officially entered World War II.
Step-by-step explanation:
President Roosevelt described the German U-boats and their crews as "the rattlesnakes of the Atlantic". This reference was made in the context of the intense submarine warfare that took place during World War II, where U-boats operated in groups known as wolf packs to destroy Allied shipping in the Atlantic. These relentless attacks by hundreds of U-boats aimed to cut Britain and Russia's Lend-Lease lifelines from the United States, contributing to what was essentially an undeclared naval war between Germany and the United States before America's formal entry into the war.