Much of the island's coastline is edged with coral reefs and cliffs, though beaches suitable for landing troops exist in the center of the west coast.

A naval coaling station was established on the island in 1905, and a battery of six 6-inch (15 cm) guns was emplaced to strengthen Guam's defenses in 1909.

A U.S. Navy captain served as both governor of Guam and commander of the naval base from 1899 onwards, though there were some elements of a civilian government on the island.

Japan gained a mandate over the islands from the League of Nations in December 1920, and they were administered by the South Seas Bureau which formed part of the Ministry of Overseas Affairs.

As a result, no improvements were made to Guam's defenses during the 1920s and 1930s, the island's coastal artillery battery was removed by 1930, and the USMC seaplane unit departed in 1931.

The South Seas Detachment was concentrated in Korea during November 1941 and, following a brief stay in Japan, sailed for Chichi-jima in the Bonin Islands late that month.

The 370-man strong 5th Company of the 2nd Maizuru Special Naval Landing Force, which was based at Saipan in the Marianas, was also assigned to join the assault on Guam.

On 17 October dependents of American military personnel on the island were evacuated to the United States by the transport USS Henderson, followed by more than 1,000 construction workers.

[5] At the outbreak of war on 8 December 1941 (local time), Guam was defended by small US Navy and USMC units as well as the Insular Force Guard.

Captain George McMillin, who was the island's governor and the overall commander of the garrison, was in charge of naval forces, Guam, which amounted to 271 personnel and four nurses.

Guam's guard ship, the USS Gold Star, had sailed to the Philippines to pick up supplies and enable the crew to buy Christmas presents and was directed to remain there.

[1]: 18  The minesweeper USS Penguin was present at the island, along with the immobile oil depot ship Robert L. Barnes,[1]: 19  and two old yard patrol boats YP-16 and YP-17.

[6][7][8] Marine barracks, Sumay, had a strength of 145 men who were organised into a company armed with rifles and a small number of machine guns.

At 08:27 Japanese land-based aircraft from Saipan attacked the Marine barracks, Piti Navy Yard, Libugon radio station, Standard Oil Company, and the Pan American Hotel.

[10] The Japanese landed about 400 troops of the 5th Defence Force from Saipan on Guam in the early morning of 10 December 1941 at Dungcas Beach, north of Agana.

The principal engagement took place on Agana's Plaza de España at 04:45 when a few Marines and Insular Force guardsmen fought with the Japanese naval soldiers.