The 1941 battle of Guam (10 December 1941) saw the Japanese overwhelm a small American garrison after three hours of fighting.
Guam had come under American control as a result of the Spanish-American War, when it was conquered by a small American naval expedition. In 1898 the army was placed under the control of the Navy Department, and a naval governor was put in place. The island was developed into a small naval base, but the American work on Guam was overshadowed by the far more determined Japanese efforts to develop nearby Saipan and Tinian.
Different sources give difference numbers for the American garrison in December 1941. The Army's Campaign in the Marianas gives a total of 153 marines, 271 US Navy personnel and 307 Guamanians, a total of 731 men. There were also three small patrol craft, including USS Penguin with two 3in anti-aircraft guns. Other sources say 430 US Marines and 180 Chamorro guards, a total of 610. The defence of Guam was commanded by Captain C. J. McMillin, governor of the island.
The Japanese invasion force was 5,400 strong. It was built around General Hori Tomitara's South Seas Detachment, which was waiting for the outbreak of war on the Bonins. It was supported by Naval aircraft based on Saipan. Reconnaissance of the island began in mid-October and by early December Japanese ships were seen in the waters around the island. The plan was to wait until the outbreak of war was confirmed. The aircraft of Saipan would then hit Guam, while the South Seas Detachment would sail from the Bonins, and land on Guam early on 10 December. Another 500 men from the 5th Defense Force on Saipan would join the invasion force.
The first Japanese air raids came on 8 December, three hours after the confirmation of war. In the first raid the Penguin was sunk and the oiler Barnes had to be abandoned. The raids were repeated on the following day, but by then the number of targets was very limited.
Landing operations began at 0230 on 10 December. The Army landed on the west coast, expecting to be able to move north to Agat, but a lack of roads meant that they had to re-embark and move north by sea.
While the Army was moving along the coast, the Navy detachment landed two miles to the north of Agana (Hagatna). This outflanked the American defenders, who were concentrated on the Orote Peninsula. Most of the fighting too place at Agana Plaza, where the first two attacks were repulsed. Despite these early successes, Governor McMillin realised that resistance was futile and surrendered after only three hours. The defenders lost 17 men, the Japanese only one, and the main Army force never got into battle, but the small naval detachment was enough to overwhelm the defenders. Guam would remain in Japanese hands until the American invasion of 21 July-9 August 1944.