Short answers, pulled from the story.
The Second Happy Time ran from January to about August 1942, during which Axis submarines sank 609 ships off the North American coast. Only 22 U-boats were lost in the same period. Historian Michael Gannon called it 'America's Second Pearl Harbor,' and the losses equaled roughly one quarter of all ships destroyed by U-boats during the entire Second World War.
German submariners called it the 'Happy Time' or the 'Golden Time.' The official German operational name for the opening attack was Operation Paukenschlag, where 'Paukenschlag' translates as 'drumbeat.'
Historian Michael Gannon placed the blame on Admiral Ernest J. King, commander-in-chief of the United States Navy. King rejected British offers to provide escort vessels and allowed the three Atlantic commands to develop their own anti-submarine tactics independently, leaving the eastern seaboard without coordinated defenses.
Operation Paukenschlag was the opening German submarine offensive along the North American coast, launched in December 1941. Five Type IX submarines carried out the first wave of attacks, with each boat carrying sealed orders to be opened after crossing 20 degrees West longitude. Reinhard Hardegen of U-123 alone sank seven ships before exhausting his torpedoes.
The introduction of an American convoy system effectively ended the Second Happy Time. The first organized US coastal convoy sailed on the 14th of May 1942 from Hampton Roads, Virginia, bound for Key West, Florida. By early July 1942, most German submarines had retreated to the perimeter of the Caribbean.
Seventy-three American tankers were lost in the first half of 1942, along with 22 British tankers in March alone. Three of the four largest Canadian tankers were also sunk between February and May 1942, triggering gasoline rationing in eastern Canada by the 1st of April.