Guam is the largest of the Marianas, 32 miles (52 km) long and 10 miles (16 km) wide. It had been a United States possession since its in 1898 until it was on 10 December 1941, following the . It was not as heavily fortified as the other Mariana Islands such as that had been Japanese possessions since the end of , but by 1944 it had a large Japanese garrison.
The plan for the invasion of the Marianas, , called for heavy preliminary bombardment, first by carrier aircraft and planes based in the to the east, then once air superiority was gained, close bombardment by battleships. Saipan, Tinian, and Guam were chosen as targets due to their size, their suitability as a base for supporting the next stage of operations toward the , and the ; the deep-water harbor at was suitable for the largest ships; and airfields for could be built from which to bomb Japan.
The was scheduled for 15 June 1944, with landings on Guam tentatively set for 18 June. The original timetable was optimistic, however. A and stubborn resistance by the unexpectedly large garrison on Saipan led to the invasion of Guam being postponed for a month.
A US naval and air bombardment lasted from 11-13 June 1944, involving 216 carrier aircraft and B-24s. On the 12th and 13th, 12 Japanese cargo ships and several fishing vessels were sunk. Battleships started shelling the island on 27 June, joined by a carrier group on 4 July, and two more on 6 July, with the loss of 16 US aircraft.
Guam, ringed by reefs, cliffs, and heavy surf, presents a formidable challenge for an attacker. the beaches and removed obstacles from 14-17 July. Despite the obstacles, on 21 July, the Americans landed on both sides of the Peninsula on the western side of Guam, planning to secure Apra Harbor. The landed at 08:29, and the landed near to the south. Japanese artillery sank 20 , and inflicted heavy casualties on the Americans, especially on the 1st Provisional Marine Brigade, but, by 09:00, men and tanks were ashore at both beaches. By nightfall, the Americans had established about 6,600 feet (2,000 m) deep. Japanese counterattacks were made throughout the first few days of the battle, mostly at night, using infiltration tactics. Several times they penetrated the American defenses and were driven back with heavy losses of men and equipment.
The had a more difficult landing on 23-24 July. Lacking amphibious vehicles, they had to wade ashore from the edge of the reef where they were dropped by their landing craft. The men stationed in the two beachheads were pinned down by heavy Japanese fire, making initial progress inland quite slow. Supply was very difficult for the Americans in the first days of the battle. Landing ships could not come closer than the reef, several hundred yards from the beach, and amphibious vehicles were scarce.
The 1st Provisional blocked off the on 25 July, and that same night Lt. General Takeshi , coordinated with a similar attack against the 3rd Division to the north. The next day, General Obata reported, “our forces failed to achieve the desired objectives.” was killed on 28 July, and Lieutenant General took over the command of the defenders. On 28 July, the two beachheads were linked, and by 29 July, the peninsula was secure.
The counterattacks against the American beachheads, as well as the fierce fighting, had exhausted the Japanese. At the start of August, they were running out of food and ammunition and had only a handful of tanks left. Obata withdrew his troops from the south of Guam, planning to make a stand in the mountainous central and northern part of the island, “to engage in delaying action in the jungle in northern Guam to hold the island as long as possible”.
After ensuring no significant Japanese forces operated in the southern portion of Guam, Major General Geiger started an offensive north with the 3rd Marine Division on the left flank, and the 77th Infantry Division on the right, liberating on the same day. The Airfield was captured on 1 Aug.
Rain and thick jungle made conditions difficult for the Americans, but after an engagement with the main Japanese line of defense around Mount from 2-4 August, the Japanese line collapsed. The 1st Provisional formed up on the left flank of the 3rd Marine on 7 August, due to the widening front and continued casualties, in an effort to prevent the Japanese from slipping through the American gaps. The Japanese had another stronghold at Mount Santa Rosa, which was secured on 8 Aug.
On 10 August, organized Japanese resistance ended, and Guam was declared secure, though an estimated 7,500 Japanese soldiers were estimated to be at-large. The next day, Obata committed at his headquarters on Mount Mataguac, after sending a farewell message to Japan.
After the battle, Guam was turned into a base for Allied operations. Five large airfields were built by the , and B-29 bombers flew from and on the island to attack targets in the Western Pacific and on mainland Japan.