In 1949 the United States had lost its nuclear supremacy when the Soviet Union successfully detonated an atomic bomb at their test site in Kazakhstan.
USSR conducts its first atomic bomb test – 1949
Several weeks after a successful Soviet atomic bomb test, Klaus Fuchs, a scientist in the U.S. nuclear program, was discovered spying for the Soviet Union.
These two events, and the fact that the USSR had access to American research information regarding the construction of the hydrogen bomb, led Truman to make this public announcement on January 31, 1950:
“It is part of my responsibility as commander in chief of the Armed Forces to see to it that our country is able to defend itself against any possible aggressor,” Truman said. “Accordingly, I have directed the Atomic Energy Commission to continue its work on all forms of atomic weapons, including the so-called hydrogen or superbomb. Like all other work in the field of atomic weapons, it is being and will be carried forward on a basis consistent with the overall objectives of our program for peace and security.”
Source: Politico