problems. Simultaneously the Korean War had made the Enewetak proving grounds rather vulunerable in addition to being very expensive in tems of military resources. Two days after he declared the national emer- gency, President Truman approved the Commission's recommendation to establish a continental proving ground at the Las Vegas bombing and gunnery range in Nevada. One of the chief advantages of the Las Vegas site was that using it posed fewer radiological hazards to Americans than any of the other sites considered. Due to the Korean emergency the Commission began planning its first continental test series on December 20, 1950, and conducted the first test explosion a little over a month later. The test series, called operation Ranger, eventually consisted of six test shots, all of them of relatively small yields, and provided important data for fission weapons and for the thermonuclear program tests scheduled for the Pacific in April.?* Quickening the Pace of Testing Meanwhile, the hycrogen banb program was floundering. Calculations by Los Alamos mathematician Stanislaw Ulam demonstrated that a proposed design for the hydrogen barb would not ignite a fusion reaction. A few weeks later Ulam suggested a new approach and by April Edward Teller had grasped the key to starting the fusion reaction. A successful experiment in the spring test series confirmed that Teller was on the right track. 15 The spring test series, called Greenhouse, was conducted in the Pacific in April and May 1951. principles for fission weapons, ‘The series was designed to test design and because of Teller's recent idea, | would be an important milestone in the development of thermonuclear