American entry into World War II in 1941 created sudden openings in strategic industries, and Rosenberg, who had been forced to live by his wits since childhood, seized an opportunity, signing on at the Hingham Shipyard south of Boston, where naval vessels were under round-the-clock construction to replenish the U.S. fleet, which had suffered heavy losses at Pearl Harbor. In his autobiography Time to Make the Donuts (2001), Rosenberg jokes that although he barely knew how to plug in an appliance when he reported for work, he was randomly assigned as an electrician, to be trained on the job. Obliged to join the United Steelworkers Union, he was elected shop delegate, an experience that helped him develop his abilities to organize and motivate people. He briefly considered a career with the union, but after the war he opted to follow an entrepreneurial inspiration.