This was the Japanese seizure of Guam island, the sole US possession in the Mariana islands group (8/11 December 1941).
The largest and most southerly of the Mariana islands group, some 35 miles (56 km) long and 9 miles (14 km) wide with an area of 210 sq miles (544 km²), more than the rest of the Mariana islands group combined, Guam has a damp climate, and receives rain every day during the wet season from July to December. The temperature ranges between 21° and 33° C (69° and 91° F). The southern half of the island is mountainous with a maximum elevation of 1,334 ft (407 m) at Mt Lamlam, while the northern part of the island is a relatively flat plateau, some 400 to 500 ft (120 to 150 m) in height, and ends in high cliffs above the shore. The island is more heavily vegetated than the other islands of the Marianas group, with the higher peaks heavily forested and the remainder covered with scrub and grassland. There are numerous streams draining to the south-east coast, and some of the low ground in the southern half of the island is swamp. The northern plateau is so heavily vegetated that it was actually less developed than the rugged southern half, where most of the cultivated land was found. The only portion of the coast suitable for amphibious landings was the south-west coast, where the offshore reef was less formidable and there were no high cliffs immediately behind the beaches.
Guam had become a US possession after the Spanish-American War of 1898. Like Wake island to its north-east, Guam was a staging post between the major US facilities on the US west coast and the Hawaiian islands in the east, and the major US possession of the Philippine islands group in the west.
Thus the Japanese had planned the seizure of Wake and Guam islands been before they turned their attention to the desirability of destroying the US Pacific Fleet at Pearl Harbor in ‘Ai’. Both islands were to be taken by Vice Admiral Shigeyoshi Inouye’s 4th Fleet based at Truk, the principal Japanese mid-Pacific naval base in the Caroline islands group.
The Americans had arranged for the evacuation of all families, dependents, and other non-essential personnel some weeks before the start of hostilities.
From March 1941 Japanese aircraft flew reconnaissance sorties over the island, and the plan for the invasion of the island had been completed by September of the same year. The unit selected for the operation was Major General Tomitaro Horii’s South Seas Detachment centred on the 144th Regiment and other units detached from Lieutenant General Takeshi Koga’s 55th Division. The maximum strength available was 4,886 men. The South Seas Detachment was concentrated in Korea during November 1941 and, after a brief stay in Japan, sailed for Chichi-jima in the Bonin islands group at a time late in the same month. The 370-man 5th Company of the 2nd Maizuru Special Naval Landing Force, based on Saipan in the Mariana islands group, was also assigned to the assault.
The Japanese forces were transported to Guam by nine transports escorted by the minelayer Tsugaru and four destroyers. The four heavy cruisers of Rear Admiral Aritomo Goto’s 6th Cruiser Division were also available to provide support if needed, and the landing force and naval units were supported by the 18th Air Flotilla, which was based at Saipan and equipped only with floatplanes.
The USA believed that it was neither possible nor practical to defend Guam. The island was not seen as being of utility in US efforts to reinforce the Philippine islands group, though it served as a refuelling point for the Pan American airline’s commercial flying boat services and was also a relay points for the Pacific Cable Company’s telegraph cable linking the Philippine islands group and the US west coast.
In 1941 the island’s defence rating was so low that the construction of new defences was not authorised and it was ordained that the island’s defenders were to destroy all facilities of military value and withdraw should war break out. Despite the island’s low priority, however, some minor steps were taken to improve the island’s defences before war broke out. A contract for small improvements to the military facilities on Guam was issued in April 1941, and work began during the following month. The Guam Insular Force Guard, which was a locally manned militia force responsible for protecting the naval base, was also slightly expanded in May.
On 17 October the US military personnel’s dependants were evacuated by sea to the USA, and these were followed by more than 1,000 construction workers. On 23 October the US Navy provided Secretary of the Navy Frank Knox with a report on the defences of Guam, and recommended against any reinforcement of an island that presented immense logistical problems of defence at a time when resources were more practically accorded to other areas. The report did also argue in favour of continuing to improve Guam’s harbour and seaplane facilities, however.
On the outbreak of war on 8 December 1941 (local time), Guam was defended by a small number of US Navy, US Marine Corps and Insular Force Guard personnel. Captain George J. McMillin, the US Navy officer who was also the island’s governor and overall commander, headed the Naval Forces, Guam, which comprised 271 largely unarmed personnel and four nurses. This force was subordinate to the Vice Admiral Thomas C. Hart’s Philippines-based Asiatic Fleet. Guam’s guard ship, the 5,066-ton auxiliary Gold Star, had sailed to the Philippine islands group to pick up supplies and was ordered to remain there. The minesweeper Penguin was at the island, however, along with the immobile oil depot ship Robert L. Barnes.
The Marine Barracks Sumay had a strength of 145 men organised into a company armed with rifles and a small number of machine guns. The Insular Force Guard comprised 246 men, most of whom had received little training. In addition to these military units, Guam’s police force (the Guam Insular Patrol) had 80 men armed only with pistols.
At 04.45 on 8 December, McMillin was informed about the ‘Ai’ attack on Pearl Harbor. At 08.27, Japanese aircraft from Saipan attacked the Marine Barracks, the Piti Navy Yard, Libugon radio station, the Standard Oil Company’s installation, and the Pan American Hotel, and also sank Penguin, which lost one officer killed and several men wounded. The air raids all over Guam continued to 17.00, being supplemented by floatplanes from the seaplane tender Kiyokawa Maru.
At 08.30 on the following day the Japanese air attacks resumed, with no more than nine aircraft attacking at any one time. The same targets were attacked, together with the Government House in Agana and several villages. That evening, a Japanese invasion fleet of four heavy cruisers, four destroyers, two gunboats, six submarine chasers, two minesweepers and two tenders departed Saipan. The Japanese landed about 400 men of the 5th Defence Force from Saipan on 10 December at Dungcas Beach, north of Agana. They attacked and quickly defeated the Insular Force Guard in Agana before advancing on Piti, and moving toward Sumay and the Marine Barracks. The principal engagement took place on Agana’s Plaza de Espana at 04.45 when a few marines and Insular Force Guardsmen fought with Japanese marines.
After token resistance, the marines surrendered at 05.45, and McMillin officially surrendered at 06.00. A few skirmishes occurred in other parts of the island before news of the surrender spread and the rest of the island forces laid down their arms. The US patrol boat YP-16 was scuttled, and YP-17 was captured by the Japanese.
Meanwhile the South Seas Detachment made separate landings at Tumon Bay in the north, near Merizo on the south-west coast, and at Talafofo Bay on the east coast.
The US Marine Corps lost five men killed and 13 wounded, the US Navy eight men killed, and the Guam Insular Force Guards four men killed and 22 wounded. One Japanese marine was killed and six wounded. The US military and civilian personnel were held for a month on the island and then shipped to detention in Japan.
By 1944 Guam was still not as heavily fortified as the other Marianas islands, such as Saipan, which had been Japanese possessions since the end of World War I, but by 1944 it had a very substantial garrison.