Which country tested nuclear weapons in the Republic of Kiribati between 1957 and 1958?

Operation Grapple was the name of four series of British nuclear weapons tests carried out in 1957 and 1958 at Malden Island and Christmas Island in the Pacific Ocean (both belonging to Kiribati). Nine nuclear explosions were initiated, culminating in the United Kingdom becoming the third recognised possessor of thermonuclear weapons.