was in part technical — the weapon design was still quite uncertain and unworkable — and in part moral: such a weapon, they argued, could only be used against large civilian populations, and could thus only be used as a weapon of genocide. Many scientists, such as Teller's colleague Hans Bethe (who had discovered stellar nucleosynthesis, the nuclear fusion that takes place in stars), urged that the United States should not develop such weapons and set an example towards the Soviet Union. Promoters of the weapon, including Teller and Berkeley physicists Ernest Lawrence and Luis Alvarez, argued that such a development was inevitable, and to deny such protection to the people of the United States — especially when the Soviet Union was likely to create such a weapon itself — was itself an immoral and unwise act. Still others, such as Oppenheimer, simply thought that the existing stockpile of fissile material was better spent in attempting to develop a large arsenal of tactical atomic weapons rather than potentially squandered on the development of a few massive "Supers". In any case, work slowed greatly at Los Alamos, as some 5,500 of the 7,100 scientists and related staff who had been there at the conclusion of the war left to go back to their previous positions at universities and laboratories. A conference was held at Los Alamos in 1946 to examine the feasibility of building a Super; it concluded that it was feasible, but there were a number of dissenters to that conclusion. When the Soviet Union exploded their own atomic bomb (dubbed "Joe 1" by the US) in August 1949, it caught Western analysts off guard, and over the next several months there was an intense debate within the US government, military, and scientific communities on whether to proceed with the far-more-powerful Super. On January 31, 1950, US President Harry S. Truman ordered a program to develop a hydrogen bomb. Many scientists returned to Los Alamos to work on the "Super" program, but the initial attempts still seemed highly unworkable. In the "classical Super," it was thought that the heat alone from the fission bomb would be used to ignite the fusion material, but that proved to be impossible. For a while, many scientists thought (and many hoped) that the weapon itself would be impossible to construct. Ulam's and Teller's contributions The exact history of the Teller–Ulam breakthrough is not completely