The Battle of Guam, in the Mariana Islands, took place in July and August 1944 during the Pacific War of the Second World War (1939-45). Guam had been US territory prior to the conflict but was occupied by the Japanese military in 1941. A massive US naval bombardment and amphibious assault eventually cleared the island of the Japanese occupiers but at the cost of around 1,700 US lives. Guam became an important naval and air base for the rest of the war, greatly helping US forces move deeper into Japan's Pacific empire and attack Japan itself.