The battle of Guam (21 July-9 August 1944) saw the Americans reconquer an island that had been in their hands before the war after three weeks of fighting, completing the conquest of the Mariana Islands.
Guam was the southern island in the Marianas group. Like Saipan and Tinian it offered several locations for B-29 airfields, but unlike those islands it also had an excellent protected anchorage that could be used by the US fleet. The island had been in American hands since the Spanish-American War, but it had fallen to the Japanese on 10 December 1941 after three hours of fighting, and had been in their hands ever since.
The original plan had been to invade Guam on 18 June, three days after the invasion of Saipan. However early on 16 June Admiral Spruance cancelled that decision, as a result of the discovery of a massive Japanese naval force heading his way. The invasion of Guam was postponed, and the fleet prepared to fight a major battle. The resulting battle of the Philippine Sea (19-20 June 1944) was a crushing defeat for the Japanese, effectively destroying the IJN's naval aviation branch. The invasion was then further postponed when the main reserve force for the invasion had to be committed on Saipan. The new reserve force, the 77th Division, had to move up from Hawaii.
The delay meant that Guam was subjected to the longest pre-invasion naval bombardment of the Pacific War, lasting from mid June, when the US fleet first appeared off the islands, to the invasion, well over a month.
Under the new plan Guam would be attacked first, with the invasion of Tinian starting three days later.
The island was protected by natural barriers. There were cliffs along much of the shore, and reefs blocking the approaches. The northern half of the island wasn't suitable for attack. The best landing points were in the south-west, south of the Orote Peninsula and north of the Piti naval station, north of the Peninsula. The island was long and narrow, with the Orote Peninsula the only major one. The southern half of the island is more rugged than the north, with a line of mountains around the southern half of the west coast. Apra Harbour, the good anchorage, was sheltered by the Orote Peninsula, while the capital city of Agana was further to the north-east along the coast.
Guam was defended by 19,000 Japanese troops under General Takashina Takeshi. There were two operational airfields on the island at the time of the invasion, one on the Orote Peninsula on the west. Takashina realised that the invasion would almost certainly come in the south-west and eight of his eleven infantry battalions were posted in that area and the other three moved slowly closer to that area as the bombardment continued.
The invasion of Guam was carried out by the newly formed 3rd Amphibious Corps (General Roy Gieger). Three forces were involved, all of which were to land on the western coast. On the left the 3rd Marine Division (General Allen Turnage) landed north of Apra Harbor. On the right the 77th Infantry Division (General Andrew Bruce) and the 1st Brigade (General Lemuel Sheperd) landed south of Apra Harbor. The two landing zones were five miles apart, and it would take four days for the two forces to join up.
The invasion began one minute ahead of schedule when the 3rd Marines landed at Asan Beach, on the American left, at 0829. After a day of heavy fighting that coast the Marines 105 dead, 536 wounded and 56 missing, the Marines had established a beachhead that was 4000 yards wide and a mile dead in most places. That night there was a disorganised Japanese counterattack, but nothing like the massive banzai charge that had been expected.
On the right the 1st Marine Brigade ran into heavy Japanese fire while approaching Agat beach, losing 350 men during the day. By the end of the day the Marines had established a food hold 4,500 yards wide and 2,000 yards deep. The major Japanese counterattack of the night came on this front, where there were three large attacks between 0230 and 0400. All three were repulsed, and as was so often the case these wasteful counterattacks achieved nothing apart from weakening the defenders.
The two beachheads remained separate for another four days, fighting largely separate battles.
On the right the 1st Brigade captured the Alifan ridgeline and established a strong right flank. One regiment from the 77th Division took over on the left, allowing the rest of the marine brigade to wheel right and advance north. They were ready to attack the Orote peninsula by 26 July.
On the left the 3rd Division had to capture a series of mountainous ridges before they reached more open ground on the Fonte Plateau. On their right they pushed south towards a junction with the 1st Brigade, leaving their lines dangerously stretched. General Takashina realised that this gave him a chance, and on the night of 25-26 July he committed seven battalions to a determined counterattack. This time the banzai attack came perilously close to success, with Japanese troops breaking into the American positions and the battle descending into a series of skirmishes scattered around the beachhead. Even so US firepower eventually won the battle, and when the fighting finished on the following day the Japanese had lost 3,500 dead (including 95% of the officers involved in the attack), the Americans 166 dead, 645 wounded and 34 missing. On the same night the Japanese troops trapped on the Orote Peninsula carried out an equally costly, but less effective attack. In one night the Japanese had thrown away a large part of their strength.
The surviving Japanese troops made the US advance as difficult as possible. It took four days for the 1st Brigade to capture the Orote Peninsula, which fell on 29 July. By now the Japanese had evacuated the southern half of the island. The Americans concentrated on taking the high ground in front of their beachheads, and made secure contact between them on 28 July. They then turned north, with the 3rd Marines on the left and the 77thj Infantry on the right. Once again the advance north was hard fought, and fairly costly, although not as bad as they might have been if the Japanese hadn't wasted so much strength on the night of 25-26 July. On the right the 77th captured the final Japanese strong point at Mount Santa Roas, while on the left the Marines fought their way to the north coast.
The advancing troops finally reached the northern end of the island on 10 August, and the island was declared to be secure. Even so Japanese troops continued to offer resistance for some time to come, and the last two didn’t surrender until 1960! The fall of Guam ended the major fighting in the Marianas, and left the islands firmly in American hands. On Guam they had lost 1,744 dead and 5,970 wounded, while the Japanese had lost over 18,000 dead and 1,250 prisoners. The Americans were now free to use Guam as a naval and air base, and strike deep into the Japanese Empire and even reach the home islands.