Final answer:
The United States was the first country to develop the H-bomb, successfully testing it in 1952 as part of Operation Ivy, marking a turning point in the nuclear arms race during the Cold War era.
Step-by-step explanation:
The development of the hydrogen bomb, often known as the H-bomb, was a pivotal moment during the Cold War era, symbolizing the advancement in nuclear weaponry beyond the atomic bombs used during World War II. The United States successfully tested the first hydrogen bomb on November 1, 1952, on the Enewetak Atoll in the Marshall Islands. This significant event was part of Operation Ivy and the code name for this test was Ivy Mike.
The test marked a substantial escalation in the nuclear arms race, leading to the Soviet Union (USSR) accelerating its own program and detonating its first hydrogen bomb on August 12, 1953. The other countries listed, including the United Kingdom and China, developed their own H-bombs later. The United Kingdom's first successful thermonuclear bomb test was in 1957, whereas China achieved this in 1967, indicating the United States was indeed the first.