Why did the United States and Soviet Union develop hydrogen bombs during the Cold War?

During the Cold War, the United States and the Soviet Union developed hydrogen bombs primarily to maintain or extend their nuclear superiority and to ensure effective nuclear deterrence against one another.12

Following the Soviet Union's successful test of an atomic bomb in 1949, the U.S. monopoly on nuclear weapons ended, forcing a shift in strategy. The United States pursued the thermonuclear bomb—a weapon with vastly greater destructive potential than fission-based atomic bombs—to regain a decisive technological advantage. President Harry S. Truman approved the project, reasoning that the Soviet Union would not be restrained by moral concerns and would inevitably develop the technology itself.21

For the Soviet Union, the development of the hydrogen bomb was a direct continuation of the arms race, aimed at achieving parity and finding a means to threaten the United States from Soviet soil. Both superpowers viewed these weapons as essential to their defense policies, eventually leading to a doctrine of mutual assured destruction, where the guarantee of overwhelming retaliation served to deter direct conflict between the two nations.13