After the lean years of the Great Depression, American workers found plenty of jobs at higher rates of pay, as the demand for workers outgrew supply in most, if not all, war industries. Women entered the workforce in numbers previously unknown, and African Americans filled many jobs closed to them just a few years earlier. Nation-wide labor unions agreed not to strike during the war in exchange for improved benefits, but small "wildcat strikes" did occur. Although wages and benefits improved during the war, workers did not necessarily experience upward mobility. They went where they were needed but could not leave when better jobs opened up elsewhere. To leave one job for another, a worker needed a release called a "Certificate of Availability" declaring the worker no longer necessary in his previous position. With the winning of the war first and foremost in the minds of the government, management, and labor, "Certificates of Availability" were not liberally issued.