Operation Battle of Guam

The 'Battle of Guam' was fought between the US and Japanese forces for the Guam, the largest of the land masses in the Mariana islands group, as part of the 'Forager' strategic undertaking (21 July/10 August 1944).

The island of Guam was a US territory which had been captured by the Japanese in December 1941 as part of the first flush of the Japanese imperialist expansion after triggering the Pacific War by the 'Ai' carrierborne air attack on Pearl Harbor in the Hawaiian islands group.

The recapture of Guam, together with the 'Forager' seizure of the other islands of the Marianas group and the 'Stalemate II' capture of Peleliu in the Palau islands group resulted in the destruction of much Japan’s naval air power at the 'Battle of the Philippine Sea' on 19/20 June 1944, and allowed the USA to establish major air bases from which it could bomb the Japanese home islands with its new strategic weapon, the Boeing B-29 Superfortress four-engined heavy bomber.

Before World War II, the government of the USA, which had gained control of Guam in the Spanish-American War of 1898, officially designated the island simply as Guam, deleting the appendages 'Island' and 'Mariana Islands', the latter to sever any identity with the Japanese-controlled islands of the South Seas Mandate given Japan after the defeat of Germany in World War I. After its capture by the Japanese, Guam was renamed Omiya Jima (Great Shrine Island) and was codenamed 'Stevedore' in US planning. Guam is located at the southern end of the Mariana islands group, with Saipan 100 miles (160 km) to the north-east and Rota island 47 miles (76 km) in the same basic direction. Kwajalein atoll in the Marshall islands group lies 1,000 miles (1610 km) to the east and Ulithi atoll in the Caroline islands group is 400 miles (645 km) to the south-west.

Guam is by far the largest island in the Marianas group with am area of 225 sq miles (540 kmē): the total land area of the group’s other 14 islands is 177 sq miles (458.5 kmē). The island is 32 miles (51.5 km) long on its north/south axis, and 10 miles (16 km) across its northern and southern lobes; across the narrow central isthmus, the island is 4 miles (6.4 km) wide. This lowland isthmus, with Agana Bay on the western coast and Pago Bay on the eastern coast, separates the island’s considerably different ends. Temperatures are moderate to hot, with a night low of 20.5° C (69° F) and a high of 33° C (91° F). The humidity is high, and it rains daily between July and December.

The island’s southern end is dominated by a rugged hill chain extending close to the western coast as far as Agana, the island’s main town. The elevations are not as great as those on Saipan, but the hills are nonetheless rugged and possess caves and ravines. Mt Lamlam near the lower end of the western coast is 1,334 ft (407 m) high. Notable peaks running to the north from Mt Lamlam are Mt Alifam at 869 ft (255 m)), Mt Tenjo at 1,022 ft (312 m)), Mt Chachao at 1,046 ft (319 m) and Mt Alutom at 1,082 ft (330 m)). The vegetation on Guam is more lush, dense and tropical than that of the chain’s other islands. The southern end is covered with sword, cogon and bunch grass as well as scrub forest, although Mt Lamlam, Mt Taene and Mt Alifan were forested with large stands of hardwoods. The southern valleys are fertile, and were under cultivation with rice and vegetables. The soil of the island’s southern half is red volcanic clay, which turns into a thick mud after heavy rains. Numerous rivers and streams flow out of the hills to the sea, and Guam is the only island in the Marianas group with rivers and streams, and is self-sufficient for water.

The largest rivers, which are Talofofo, Ylig and Pago, flow toward the eastern coast. A large swamp is located inland of the south-eastern coast and another swampy area is sited on the western side of the isthmus along the Agana river on the eastern side of Agana. Much of the isthmus is covered with coconut palms.

The island’s northern portion comprises a limestone plateau between 400 and 500 ft (122 and 311 m) in height above sea level. This plateau has rolling terrain and is broken by three hills, Mt Barrigada at 674 ft (205 m), Mt Santa Rosa at 870 ft (265 m) on the central upper part of the eastern coast, and Mt Machanao at 610 ft (186 m) at the northern end at Ritidan Point. The vegetation is extremely dense, rendering off-trail movement almost impossible in many areas. This vegetation consists of tropical forest, underbrush and undergrowth. The higher hilltops are barren, and are covered only by sword grass and scrub brush. Some cattle ranching was undertaken in the north.

The northern coast, between Tumon Bay on the western coast just to the north of the isthmus, around the northern end, and down the eastern coast as far as Pago Bay, is edged with limestone cliffs between 100 and 600 ft (30.5 and 183 m) high, and narrow coral reefs fringe this coast. The south-eastern coast is wholly different. It too is fringed by a narrow coral reef, but the cliffs are lower and there are breaks in these. The southern and south-eastern coasts are beaten by a heavy surf, and the former has rugged offshore reefs extending outward for more than 2 miles (3.2 km). Reef-surrounded Cocos island, 1.5 miles (2.4 km) long. and lying about 1.5 miles (2.4 km) off Guam’s south-western end. The south-western coast is also edged by low cliffs rising directly into the hills and possessing a very poor road system.

Only a 15-mile (24-km) stretch of the central portion of the lower western coast had suitable beaches with narrow, low reefs, and of sufficient width and depth for amphibious operations. The reefs are between 25 and 700 yards (23 and 640 m) wide, but there are numerous breaks. Extending from the centre of this portion of the coast is the Orote peninsula. Apra Harbor lies on the peninsula’s northern side and Agate Bay on its southern side. The peninsula is 4 miles (6.4 km) long and is about 880 yards (805m) wide near its base, where there were rice paddies, and widens to 1.75 miles (2.8 km) before tapering gradually to a point. It is faced with cliffs between 100 and 200 ft (30.5 and 61 m) in height on all sides. Most of the peninsula was covered with scattered palms and scrub brush, except in the area around the Orote airfield on the seaward end. A mangrove swamp was located near the coast on the peninsula’s northern shoulder. Off the peninsula’s western end is Orote island and off its south-central side is Neye island. These are both small rocky islets less than 100 yards (91 m) offshore. The pre-war US Marine Corps barracks were located on the high ground above the south-eastern side of Sumay, itself located on the north central side of the peninsula along with the cable station and Pan American airline’s flying boat station. A dredged boat and seaplane channel protected by breakwaters provided access to Sumay, which was the island’s second largest town with 2,000 inhabitants.

Apra Harbor on the peninsula’s northern side does not extend to the shore as the shore is barred by a broad reef, but nonetheless offers the best protected harbour in the Mariana islands group. Located in the centre of the harbour, 1,200 yards (1100 m) to the north-east of Sumay were the crumbled ruins of a small Spanish-era fort, Fuerte de Santa Cruz, perched on a low reef and often awash by the sea; this possessed no military value. Piti Navy Yard was located at the northern end of Apra Harbor with Tepungan village 400 yards (365 m) to its north-east. There a 500-yard (460-m) causeway connects the mainland with the boomerang-shaped Cabras island, which is 2 miles (3.2 km) long and protects the harbour’s northern side. The submerged Luminao Reef extends westward from the island’s seaward end, on which a breakwater 1 mile (1.6 km) long, 5 ft (1.5 m) above high-tide level and 36 ft (11 m) wide, had been built in 1941 using limestone blocks quarried on Cabras island. A pier 800 yards (730 m) long extended to the south-west into the harbour from Cabras island’s seaward end and protective jetties and fuelling facilities ran along the island’s southern shore. The contract labourers employed to the extent of more than 1,000 men had been evacuated before December 1941. The planned 4,500-ft (1370-m) Orote airfield had been staked out, but was then completed by the Japanese.

The US forces selected as their main landing beaches areas on each side of the Orote peninsula. To the south of the peninsula in Agat Bay were the Agat beaches near the village of Agate on their northern edge and Bangi Point on their southern edge. Two islets, Yona and Alutom, lie off Bangi Point. About 7 miles (11.25 km) to the north of the Agat beaches were the Asan beaches centred on the village of Asan. The beaches were bounded by Adelup Point on their eastern side and Asan Point on their western side. The narrow coastal plain behind both of the beaches were covered with dry rice paddies.

In US hands, Guam was [primarily a naval station, albeit only of limited capability, but it essentially encompassed the entire island and its 3-mile (4.8-km) territorial limits. A navy yard was established at Piti, the port of entry, in 1899 and a marine barracks at Sumay in 1901. The navy proposed the construction of defenses and in 1905 established a coaling station at Piti. In 1908 it was decided to concentrate on the development of Pearl Harbor, but six 6-in (152-mm) guns were emplaced in 1909. Other proposals for Guam’s defense were made before and after World War I to protect the route to the Philippine islands group, but little was done. In March 1921 a marine seaplane unit was stationed on the island as the first marine aviation unit to serve in the Pacific, but this was withdrawn in February 1931. The 1922 Washington Navy Conference, at which both the USA and Japan agreed not to fortify their possessions in the western part of the Pacific Ocean, was enacted and was to have an impact on Guam’s future military development. In the following years little was done to develop the naval station or improve the island’s defences. The coast-defence guns were removed by 1930. There were no airfields on Guam, although at least two were planned. The navy renewed its efforts, after Japan withdrew from the League of Nations, to gain approval for the development of defences on Guam, but was told in 1938 that it would be inappropriate to make further requests of the Congress. In 1939 the Hepburn Board urged that Guam be developed as an advanced fleet base, and in 1940 the Greensdale Board recommended a more modest base development, but neither gained much for the island. In 1941 Guam was given a Category 'F' defence rating, meaning that no new defences would be established and existing naval forces, being as limited as they were, would destroy facilities and materials to prevent them from falling into any enemy hands. The Congress also feared that any effort to develop defences on Guam would serve only to provoke Japan into rash action. Thus Guam’s fate was sealed. Some limited base improvement construction was approved and contract work began in May 1941: this included fuel tanks on Cabras island, road improvements and extension around the island, and building the breakwater on the northern side of Apra Harbor.

Guam’s exports, mainly copra and coconut oil, were only one-sixth of its imports. In 1936, Pan American Airways established a flying boat station at Sumay on the route liking San Francisco and Hong Kong via Manila. Communications were maintained via the navy radio stations and a commercial cable station was established in 1903 by the Pacific Cable Company.

The island’s administrative centre, Agana sits on Agana Bay on the western side of the isthmus, and in 1941 possessed a population of 12,550 persons. Numerous villages were scattered about the island, mainly along the coast. About 85 miles (137 km) of improved road allowed access to most areas, and were connected by unimproved roads and jungle tracks in other areas. Most of the coast was fringed by roads and a cross-island road extended across the isthmus.

In 1941 the island’s population was 23,400 persons, which included 21,500 native Chamorros, 800 non-natives (US, Chinese, Japanese, etc.), and more than 400 navy and marine personnel. The official language was English, but Chamorro was retained and spoken at home, and some Spanish was still spoken. The Chamorros were US nationals rather than US citizens.

By executive order in January 1941, foreign warships and ships of commerce were not permitted to enter the Naval Defense Sea Area and Naval Air Space Reservation 3 miles (4.8 km) round the island, but some exceptions were granted. Before this, Japanese trading schooners were permitted to visit the island. In March Japanese aircraft, later learned to have been photo-reconnaissance machines, were seen overflying the island, and Japan apologised. In April plans were announced to expand the naval station and improve the harbour, and the same month saw the slight expansion of the Guam Insular Force Guard, a Chamorro-manned militia-type naval station guard originally formed in 1901. No other attempts were made to fortify the island for fear of further complicating ongoing negotiations with Japan. With growing fears of war, 104 US dependents and almost 1,000 construction workers were evacuated to Hawaii and the US mainland during October. More war warnings were issued, and Japan had already made the decision to take the island.

In December 1941, Naval Forces, Guam, under the command of Captain George J. McMillin, comprised the Administrative Group, Navy Hospital, Navy Radio Stations at Agana and Libugon, and the Navy Yard at Piti plus the marine barracks at Sumay. There were 271 US Navy personnel, including Chamorro messmen and bandsmen, and 122 marines. At 04.45 on 8 December (7 December in Hawaii) McMillan was notified of the Japanese attack on Pearl Harbor. Warplanes of the Imperial Japanese navy air force from Saipan attacked the marine barracks and Pan American Hotel at 08.27 and the minesweeper Penguin was damaged off of Orote Point and scuttled. Civilians were evacuated from Agana and Sumay while some 50 Japanese nationals were arrested and detained in the Agana jail. A second attack followed at 17.00. That night nine Saipan native infiltrators landed by canoe at Ritidian Point on the northern end and three were apprehended. At 08.30 on 9 December air attacks resumed. The marines, naval station personnel and Penguin's crew members dug in near their barracks on the Orote peninsula. Other naval craft included the oiler Robert L. Branes, which was damaged and captured, the patrol vessel YP-16, which was burned, and the patrol vessel YP-17, which was damaged but captured. The cargo ship Gold Star, which was the Guam station ship, was in the Philippine islands group at this time. The 246-islander Guam Insular Force Guard, led by a few marines, secured government buildings in Agana, and the 80-islander Guam Insular Patrol, the island’s police force with 29 marines assigned to it, was stationed in villages across the island.

The 370-man 5th Company of the Maizuri 2nd Special Naval Landing Force from Saipan landed on Dungcas beach to the north of Agana at 02.15 on 10 December, and attacked and captured the Insular Force Guard in Agana. The Imperial Japanese army’s 4,886-man South Seas Detachment, from Haha Jima and commanded by Major General Tomitaro Horii, was centred on the 144th Regiment and had been organised specifically for the seizure of Guam and later of Rabaul on New Britain. It landed at the same time. One battalion landed at Turnon Bay north of Dungcas Beach and moved south toward Agana, another battalion landed at Talofofo Bay on the eastern coast and moved to the north-west, and a third battalion landed on the south-western coast near Merizo and moved to the north to attack the marines at Sumay. After token resistance, McMillin surrendered at 07.00 on 10 December, making Guam the first piece of US territory to fall into Japanese hands. Marine losses were five men killed and 13 wounded. The navy’s and Guam Insular Force Guards' losses were respectively eight and four men killed, and these two elements also suffered 22 men wounded. About 30 civilians were killed by strafing and bombing. The Japanese losses were one man killed and six wounded. The few remaining US civilians were interned and later exchanged with diplomatic personnel along with four US Navy nurses.

The Chamorros suffered heavily under Japanese rule. Only Japanese was taught in schools and rationing was instituted. The Japanese South Seas Development Company took control of all of Guam’s business enterprises. Many people died or otherwise suffered from malnutrition and limited medicines. Group punishment was inflicted when individuals were accused of infractions of the occupation regulations. The Japanese built two small airfields, a 4,500-ft (1370-m) strip on the Orote peninsula and a 5,000-ft (1525-m) strip at Tiyan (also known as Agana) so,e 2.5 miles (4 km) to the east of Agana: this latter had not been completed by the 1944 US attack. An airfield was cleared, but no other work accompanied, at Dededo 3 miles (4.8 km) to the north-east of Tiyan and near Turnon Bay on the western coast. Chamorros, including women and children, were impressed as forced labourers for the construction of airstrips and fortifications. The Japanese performed little other military construction on the island, but instead concentrated on the exploitation of the island’s resources. When the reinforcement of the island began in 1944, the schools and churches were closed, more restrictive rationing was imposed, and punishments were made still harsher.

With the fall of the Marshall islands group to the S 'Galvanic' and 'Catchpole' operations, the Japanese began to reinforce all their garrisons in the Mariana islands group to a total of 11,500 mens. Lieutenant General Takashina Takesi’s 29th Division (18th, 38th and 50th Regiments) arrived on Saipan from Manchukuo via Japan in February 1944 and then moved to Guam during the following month, though the 50th Regiment was moved on to Tinian. Although it was planned to send the 13th Division from China to Guam in September 1943 to counter the growing US threat in the Pacific War, only a 300-man detachment was sent. Almost half of the 18th Regiment was lost when its transport was torpedoed on its way from Japan, but it was rebuilt and the 1/18th Regiment remained on Saipan. The division doubled as the Southern Marianas Army Group. which was charged with the defence of Guam and the small Rota island. Tinian was transferred to its command on 7 July, before the fall of Saipan. A reinforcement unit, the 6th Expeditionary Unit, arrived in April and was reorganised into the 48th Independent Mixed Brigade of four battalions under the command of Major General Shigematsu Kiyoshi and the 10th Independent Mixed Regiment, the latter less the 1/10th Independent Mixed Regiment on Rota. There were also anti-aircraft units and three tank companies with 33 to 38 tanks. Imperial Japanese navy personnel totalled some 7,600 men of the 54th Guard Force, 60th Anti-Aircraft Defence Unit, and marooned sailors and air service troops organised for ground combat. The Japanese concentrated most of the [w]38th Regiment on Agat Bay, the 54th Guard Force on the Orote peninsula, part of the 18th Regiment near Apra Harbor, and most of the 48th Independent Mixed Brigade at Agana Bay. Elements of the 10th Independent Mixed Regiment were positioned around the southern and eastern coasts. Lieutenant General Hideyoshi Obata, commanding 31st Army headquartered in the Palau islands group and to which the forces in the Marianas were subordinated, was stranded on Guam when the US forces landed, and continued to direct the overall defense of the Mariana islands group, but left the tactical defence of Guam to Takeshi.

The recapture of Guam was essential to the US advance across the Pacific not only as a point of honour but because of the need of the US forces for an advanced navy operating base and for a location on which to build airfields on which to base B-29 bombers. Guam was scheduled to for assault on 18 June 1944, but this was delayed until 21 July for several reasons. It was discovered that a Japanese fleet was approaching the Mariana islands group from the Philippine islands group for the 'A' naval offensive that led to the 'Battle of the Philippine Sea' so the landing was postponed so as not to endanger the Southern Attack Force (Task Force 53) heading for Guam. With the 'Battle of the Philippine Sea' over and the Japanese naval threat removed, another delay was imposed because of the Japanese resistance on Saipan, which was stronger than had been expected. This required commitment on that island of the Expeditionary Troops Reserve, Major General Ralph C. Smith’s (later Major General George W. Griner’s) 27th Division. It was originally planned that this formation would be used on Guam alongside the marines. Fearing that additional reinforcement might be required on Saipan, the 1st Provisional Marine Brigade, slated for the assault on Guam, was retained in the area as the Expeditionary Troops Reserve while Major General Allen H. Turnage’s 3rd Marine Division withdrew to Eniwetok to await the invasion of Guam. In the meantime it was decided to release the General Reserve at Hawaii, Major General Andrew D. Bruce’s 77th Division, to the Southern Troops and Landing Force for use on Guam. The entire division could not deploy immediately, so it was decided to delay the invasion until the whole division had arrived at Eniwetok. With Saipan secured it was finally decided to assault Guam on 21 July.

The 54,690 men of the Southern Troops and Landing Force (Task Group 56.2) and III Amphibious Corps, under the command of Major General Roy S. Geiger, included Turnage’s 3rd Marine Division (3rd, 9th, 12th [artillery], 19th [engineer] and 21st Marines), Brigadier General Lemuel C. Shepherd’s 1st Provisional Marine Brigade (4th and 22nd Marines), Bruce’s 77th Division (305th, 306th and 307th Infantry), the III Amphibious Corps Artillery. The l77th Division’s 305th Infantry was initially attached to the 1st Provisional Marine Brigade as its reserve. The 26th Marines were detached from Major General Keller E. Rockey’s 5th Marine Division in the USA and deployed as the Expeditionary Troops floating reserve, but it did not land and was redeployed to Hawaii at the end of July.

Both the 3rd Marine Division and 1st Provisional Marine Brigade undertook rehearsals on Guadalcanal, while the 77th Division did the same in Hawaii. The marine formations staged through the Marshall islands group at Kwajalein atoll, cruised off Saipan in case it was required ashore, then assembled at Eniwetok early in July to await the arrival of the 77th Division later in the month. Between 11 and 18 July the Southern Attack Group departed Eniwetok and arrived off Guam’s western coast between 20 and 21 July. Guam had already been subjected to air and naval gunfire bombardments, and now underwent a 13-day air and naval bombardment immediately before the landings on 21 July, when the marines began to come ashore at 08.30.

The 3rd Marine Division landed on the Northern, or Asan, Beaches between Adelup and Asan Points and centred on the town of Asan on Guam’s west central coast. The 3d Marines landed on Beaches Red 1 and Red 2. The 21st Marines landed on Beach Green in a column of battalions, and the 9th Marines hit Beach Blue just to the north-east of Asan Point. The 1st Provisional Marine Brigade came ashore on the Southern, or Agat, Beaches some 7 miles (11.25 km) to the south-west of the Northern Beaches, from which they were separated by Agana Harbor and the Orote peninsula. The 22nd Marines landed on Beaches Yellow 1 and 2 adjacent to Agat town. The 4th Marines landed on Beaches White 1 and 2 to the north of Bangi Point. The 305th Infantry came ashore on W-Day and W+l, the 306th Infantry landed on W+2 on the Beaches White, and was followed by the 307th Infantry on W+3 on the same beaches as the rest of the 27th Division. The 305th Infantry was returned to the control of its parent division on 24 July.

The Japanese resistance was stiff on both sets of beaches, which are dominated by high ground, and the marines were able to establish only shallow beach-heads by the fall of night. The Japanese counterattacked both beach-heads, the first of many such efforts, and while small elements managed to infiltrate the front lines, the attacks failed to achieve significant penetrations. The US progress was slow for the first few days as the marines fought to link the two beach-heads and push inland onto the high ground. Cabras island was secured on 24 July and the Orote peninsula was sealed from the mainland during the next day. The peninsula was cleared by the 1st Provisional Marine Brigade on 29 July, 2,500 Japanese being killed, while the 77th Division took over the right flank and the 3rd Marine Division continued to advance on the left. Marines from the northern beaches linked with soldiers from the southern beaches at 17.45 on 28 July.

After their linking, the US forces concentrated on their drive to the north. The 77th Division undertook a reconnaissance in force of the southern portion of Guam, but found no organised resistance as most of the defenders had withdrawn to the north. On 31 July, the drive to the north began with the 3rd Marine Division on the left and the 77th Division on the right while the 1st |Provisional Marine Brigade secured the beach-head and patrolled the southern area of Guam. Progress was slow, but on 6 August the US forces approached the island’s wider northern lobe. The 1st Provisional Marine Brigade entered the line during that night on the left of the 3rd Marine Division, which was now in the centre. The US forces closed on the last Japanese centres of resistance, and the army troops overran the strongpoint on Mt Santa Rosa on the upper east coastern on 8 August. The marines reached Ritidian Point on the island’s north-western corner and the army’s soldiers Pati Point in the north-east on 10 August. Guam was declared secure at 11.31.

Almost 11,000 Japanese bodies were counted, but many more had been buried and remained undiscovered. Thousands of Japanese were still hiding in the island’s forested northern hills, and the mopping-up process continued long after the island had been declared secure. By the end of August 1945, a total of 18,400 dead had been counted, 1,250 prisoners taken, and some 500 Japanese civilians interned. The last organised group of Japanese, totalling 46 men, surrendered on 11 September 1945. Marine and US Army patrols hunted down scores of Japanese hold-outs with the aid of the marine-advised Guam Combat Patrol and Guam Police long after the end of the war in September 1945. Dozens of Japanese were still surrendering years after the war, with the last believed to have yielded himself in 1960, until one more man surrendered in 1972. The US casualties were more than 1,700 men killed and almost 6,000 wounded.

Orote Airfield was rebuilt with its runway extended to 5,000 ft (1525 m), and became operational on 29 July allowing its use by US Navy and US Marine fighters supporting the ground troops. The North and Northwest Airfields, each with two 8,500-ft (2590-m) runways for B-29 operations, were built near Ritidian and Pati Points. There had become operational in February and June 1945 respectively, and it was from North Field that the first B-29 raid on Japan was launched from the Mariana islands group on 24 February 1945. The 7,000-ft (2135-m) Agana Field was built over the old Japanese Tiyan strip to the east of Agana and used by transport aircraft. A second 6,000-ft (1830-m) runway was later constructed. The 7,000-ft (2135-m) Harmon Field, which had initially been designated Depot Field, was built to the north of Agana on the site of the cleared Japanese strip. It became the location of the XXI Bomber Command’s headquarters from a time early in December and then of the 20th Atmy Air Force’s headquarters from July 1945.

The development of port facilities at Apra Harbor was given the highest priority. Most existing facilities had been destroyed and the towns of Agana, Sumay and Piti levelled. Piers were built on Cabras island and the bottom was dredged to make it usable by deeper-draught ships. Early in October a typhoon struck Guam and much of the port construction completed to that date was destroyed but was soon rebuilt. The breakwater was extended to 17,000 ft (5180 m) the better to protect the harbour. The Naval Operating Base, Guam became a major cargo port, repair facility and submarine base. An extensive island-wide permanent road construction project was also undertaken. Among the road projects was a 12-mile (19.3-km) road, surfaced with packed coral, connecting Agana and Sumay: this became the first four-lane road in the Pacific. Numerous supply depots, hospitals, and troop camps were built. Together with Okinawa, Guam became a major staging base for the planned 'Olympic'' invasion of Japan. Housing was built for 15,000 local civilians.

The nearby and comparatively smalv Rota and Pagan islands had been occupied by Japanese forces, but were not assaulted by US forces.

Rota island, whose US codename was 'Stimulate', is 47 miles (76 km) to the north-east of Guam and 69 miles (111 km) to the south-west of Saipan. It is the most southerly unit of the Mariana islands group, and the nearest of the Japanese-mandate islands to the US territory. Rota is 10.5 miles (16.9 km) long on its north-east/south-west axis, 3 mile (4.8 km) wide, and has an area of 38 sq miles (94 kmē). It is rugged and mountainous, with Mt Manria on the south-western end rising to a height of 1,625 ft (495 m). It has a rugged coast faced with cliffs and possesses no harbour, but the open Sosanyaya Bay indents the island’s southwest end. The island’s population was 394 persons, most of them in the main village of Songsong, sand was involved primarily in fishing and sugar cane growing on the southern coast. It was at Rota that the Japanese invasion force for Guam arrived on 9 December 1941 after making passage from Haha Jima and Saipan.

The Japanese built an airfield near the north-eastern end of Rota before the US 'Forager' descent on the Mariana islands group. The 50th Regiment was earmarked to move to Rota on 15 June as the island’s garrison, but the appearance of the US invasion force for Saipan persuaded the Japanese to retain it on Tinian. The 1/10th Independent Mixed Regiment was instead sent from Tinian to garrison Rota on 23 June. The 3/18th Regiment soon followed as the basis for a counter-landing force for Saipan or Guam, but was returned to Guam on 19 June as sea conditions and US patrols made the plan impractical. Rota was subordinate to the Southern Marianas Army Group's 29th Division on Guam until this island’s fall on 10 August, and then fell under the control of Pagan island. The garrison of 1,000 Imperial Japanese army troops, 500 Imperial Japanese navy air force personnel, and 500 labourers then sat out the rest of the war.

Pago island is some 180 miles (290 km) to the north of Saipan. To its south is tiny Alamagan island and to its north Agrihan (or Agrigan) island. Pagan is the fifth island from the northernmost end of the Mariana islands chain, and is 2.5 miles (4 km) wide across its larger northern lobe and 8 miles (12.9 km) long. The smaller southern lobe is connected to the north by an isthmus 1 mile (1.6 km) wide. The island possesses three active volcanoes reaching a height of 1,870 ft (570 m) in the case of Mt Pagan in the north end and on the southern lobe another two reaching to heights of 1,883 and 1,798 ft (574 and 548 m), but despite this fact was populated. A narrow plain is situated on the island’s central isthmus, but the island is divided by a series of cross-island cliffs. Its coast is rugged, rocky and lined with cliffs. The only landing sites are at Apaan Bay on the western coast, on which the main settlement of Shomushon is located, and an area on the eastern side.

The Japanese had built an airfield on the northern end of the narrow isthmus before the start of 'Forager', and the 9th Independent Mixed Regiment arrived from Japan on Pagan with 3,500 men to garrison the island in May 1944; there were also 800 Imperial Japanese navy personnel, mainly air service, and 1,000 labourers. The regiment was commanded by Colonel Umehachi Amau, who became the senior military commander remaining in the Mariana islands group after the Japanese loss of Guam.

Air attacks on Rota and Pagan began on 11 June and continued intermittently until the war’s end as both islands were bypassed by the US forces. The garrison of Rota surrendered on 2 September 1945 to a marine officer, Colonel Howard N. Stent, from Guam, and the garrison of Pagan surrendered to US Army representatives from Saipan on the same date.

A small US Navy installation was established on each of the islands after the war.