After the war, Teller returned to academic life, but he continued to passionately advocate for the development of the hydrogen bomb. He faced opposition from some scientists who questioned the morality and practicality of such a weapon.

The Soviet Union's successful test of an atomic bomb in August 1949 heightened the urgency surrounding the hydrogen bomb project in the United States. President Truman ordered its development soon after.

Teller returned to Los Alamos to lead the hydrogen bomb development effort.

A breakthrough design, known as the Teller-Ulam design (although Stanislaw Ulam's contribution was significant and arguably foundational), provided a practical pathway to creating a hydrogen bomb. This design involved using the radiation from a fission bomb to compress and ignite a fusion fuel capsule.

The first successful test of a hydrogen bomb (Ivy Mike) demonstrated the viability of the Teller-Ulam design. The test yielded a massive explosion far exceeding the power of atomic bombs.