Operation Grapple X was the code name for the United Kingdom's first successful test of a hydrogen bomb, conducted at Kiritimati in the Pacific Ocean. This test was part of a series of nuclear tests aimed at enhancing the UK's nuclear arsenal in line with its aspirations as a leading global power during the Cold War. On November 8, 1957, the British government executed the detonation, marking a new phase in its military capabilities. The operation involved extensive planning and collaboration among military and scientific communities, with the detonation itself designed to assess the potential yields and effects of hydrogen bombs.

The Grapple series, which included several tests, was particularly noteworthy because it reflected the escalating arms race in the wake of World War II, with both the United States and the Soviet Union having already established significant nuclear arsenals. By successfully developing and testing a hydrogen bomb, the UK aimed to solidify its position among the nuclear powers and bolster its defense strategies. The test also acted as a demonstration of technological prowess and military readiness.

Kiritimati, also known as Christmas Island, was chosen for its remote location, minimizing the risk of affecting civilian populations. The detonation produced a fireball that could be seen from miles away and caused considerable destruction in the surrounding environment, albeit the full ecological impact took years to assess. The operation was not only a technical achievement but also a pivotal moment in international relations, intensifying fears of nuclear proliferation across the globe.