Background [edit]. Guam is the largest of the Marianas, 32 miles (52 km) long and 10 miles (16 km) wide. 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.ackground [edit]. Guam is the largest of the Marianas, 32 miles (52 km) long and 10 miles (16 km) wide. 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.