Guam, at 212 square miles (543 square kilometers), is the largest island of the Marianas, with a length of 32 miles (52 km) and a width ranging from 12 miles (19.31 km) to four miles (6.44 km) at different points of the island. It had been a United States possession since its capture from Spain in 1898 until it was captured by the Japanese on 10 December 1941, following the attack on Pearl Harbor. During the Japanese occupation of Guam, it was not as heavily fortified as the other Mariana Islands such as Saipan that had been Japanese possessions since the end of World War I. But by 1944, Guam had a large Japanese garrison.