On 11 December 1941, Hitler declared war on the United States. Dönitz ordered immediate offensive operations off the East Coast of the United States. Codenamed Operation Drumbeat ("Unternehmen Paukenschlag"), the U-boat fleet inflicted the largest naval defeat on the United States Navy in history. Though few in number at the beginning—just five—the U-boats pressed home attacks close to the shore, from Newfoundland to the Gulf of Mexico. The American failure to initialise a blackout, ship captains' insistence on following peace-time procedures, and lack of effective naval defences contributed to high losses. 397 ships were sunk during Drumbeat. The Germans termed this period, the "Second Happy Time." Topp crossed the Atlantic and joined Wolfpack Zieten.