By May 1939, the Japanese had laid claim and occupied the Paracel and Spratly Islands. During World War II, the Empire of Japan used the islands in the South China Sea for various military purposes and asserted that the islands were not claimed by anyone when the Imperial Japanese Navy took control of them. Historical accounts note that at least France had controlled some of the features in the region during the 1930s. After the war, Imperial Japan had to relinquish control of the islands in the South China Sea in the 1951 Treaty of San Francisco which, however, did not specify the new status of the islands. The People's Republic of China made various claims to the islands during the 1951 treaty negotiations and the 1958 First Taiwan Strait Crisis.