To support air offensives against, and maintain surveillance over, the by-passed Japanese bases in the Marshalls and the Carolines, an advance air base, with minor fleet facilities, was established on Kwajalein Atoll, in the Marshall Islands. Complete facilities were to be provided for the operation of landplanes and seaplanes.

    Kwajalein Atoll is 66 miles long and has a maximum width of 18 miles. More than 80 islands and islets lie along the partially submerges reef, which surrounds a lagoon of about 655 square miles.

    Land areas large enough to be developed are found at only three points on the atoll, Kwajalein and neighboring islands at the southeastern end, Roi Islands and nearby islands at the northern end, and Ebadon Island at the western end. The southern islands are covered with a dense growth of coconut trees and smaller vegetation; the islands to the north are wooded.

   The Kwajalein area includes Kwajalein Island and the islands on the reef for 12 miles to the north and 10 miles to the northwest. Ebeye Island lies 2½ miles north of Kwajalein, and is separated from the latter by an unbroken reef. It is 1770 yards long and 230 yards wide throughout most of its length. Kwajalein Island, crescent-shaped and open to the lagoon on the northwest, is about 3 miles long and varies in width from 1,000 to 2,500 feet.

   In the Roi area, considerable land exists only on Roi and Namur islands. Roi is 1250 yards long 1170 yards wide. Namur, to the east of Roi and connected with it by a narrow strip of land, is 890 yards long and 800 yards wide.

    Kwajalein Atoll was highly developed as a military base by the Japanese. A major air base existed on Roi Island, and on Namur, connected by a causeway to Roi, were barracks, warehouses, a radio station, and a 450-foot L-shaped pier extending into the lagoon. Kwajalein Island contained many buildings, some of which were used as warehouses for a supply center. A 2,000 foot pier extended from the lagoon side of the island. An airstrip, 400 by 5,000 feet, had also been completed. Ebeye Island was the site of a seaplane base with hangars, two ramps, and an L-shaped pier.

    On January 30 and 31, Kwajalein Atoll was subjected to heavy surface bombardment and air attack prior to landings by the Fourth Marine Division on Roi and Namur Islands and by the Seventh Army Division on Kwajalein, on February 1. The 121st Construction Battalion accompanied the first waves of Marines landing on Roi and Namur, as shore parties for combat teams. By February 2, Roi and Namur had been secured despite strong Japanese counterattack on Namur. Kwajalein Island was under Army control by February 5, and the entire atoll was secured by February 8.

Roi and Namur.— the uprooting of all vegetation and the almost-complete destruction of enemy facilities by the assault force caused a tremendous accumulation of debris. Before any progress could be made in setting up camp and storage areas, beach development, or airfield construction, it was necessary for the 121st to remove the debris as quickly as possible.

    On February 5, the battalion was ordered to consolidate and begin work on the Roi Island airfield, the 109th Battalion arriving the next day to assist on the project. The 121st, which was attached to the Marine division, had only 20 per cent of its equipment; consequently, the major portion of the heavy construction fell upon the 109th.

    Of the three Japanese oil-surfaced airstrips, the runway, measuring 300 by 4,300 feet, was the first to be reconstructed. The existing strip was ripped up and resurfaced. The first fighter squadron arrived on February 13 and operated form the base as construction continued.

    By February 12, most of the Marine division had departed and the remainder, with the 121st Battalion, were preparing to leave. On February 13, Japanese bombers launched a heavy attack against Roi, setting fire to a bomb dump. In the resulting destruction, the 109th Battalion suffered 102 casualties and lost 75 per cent of its material and 35 per cent of its equipment. The 121st’s losses were 55 casualties, equipment was transferred to the 109th to help replace losses from the attack.

    The 109th continued the airfield construction. The east-west Japanese runway, lately used as a storage area, was reconstructed by May 1944, with a 2,200-foot taxiway, and a concrete parking area was also provided. The third Japanese runway was then resurfaced for an additional repair and parking area. Including fighters, light bombers, and patrol planes, 100 planes were now based on Roi. The field was commissioned on May 15, and from it daily missions operated against Japanese installations on Wotje, Jaluit, and Truk.

    On March 5, 1944, a detachment of 15 officers and 487 men of the 95th Battalion arrived on Namur, their major assignment being the construction of an aviation supply depot, which was commissioned on June 10.
Roi was strategically located for use as a center to supply aviation materials to bombing missions against islands of the Marshalls and Marianas. Considerable amounts of supplies from this base also were used in support of the Marianas landings. 

   The 95th also erected a 4,000 barrel tank farm, with necessary piping; provided hospital facilities at three dispensaries, having a combined capacity of 300 beds; and erected housing accommodations of floored tents, Quonset huts, and barracks for all military activities.

   As there were no deep-water waterfront facilities in the area, a 4-by-30-pontoon pier was assembled and a Japanese L-shaped pier, 450 by 33 feet, was used extensively to unload supplies from small craft. Repair installations consisted of overhaul shops for small-boat motors and a 4-by-15-pontoon drydock for 100-ton capacity for aircraft rescue boats, picket boats, and LCM’s. Connecting causeways and a perimeter road for each island were also constructed.
Coral found on the islands was not of proper quality for use in surfacing, due to the large percentage of coarse sand, which resisted binding, consequently coral for surfacing purposes was taken from the lagoon. Native woods were used for minor construction, and native labor was employed on clean-up and sanitation details.
CBMU 590 arrived in June 1944 to relieve the construction battalions and continue maintenance.

 Kwajalein Island.—The 74th and 107th Battalions reported on Kwajalein Island in March 1944, the 74th setting up headquarters on near-by Berlin, or Gugegwe Island. This island had apparently been used by the Japanese as a supply base and for the repair of small craft. A marine railway with a capacity of 250 tons had been damaged by shelling, but was restored to usefulness for small-boat repair. A concrete pier, almost completed by the Japanese before our occupation, proved small but adequate, when finished for use of the boat pool. Berlin Island then became the site of shops for small-boat repair and overhaul, and of the Seabees camp.

    A rock-crushing plant had been set up by the Japanese on Berlin Island and, although damaged, was salvageable for use in connection with the development of crushed coral for airstrip and road surfacing. The Seabees then rebuilt the existing Japanese runway on Kwajalein Island to provide a 6300-foot coral-surfaced strip with two 80-foot taxiways and 102 hardstands for heavy bombers. One hangar with minor aircraft-repair installations, was erected, and more than 12 miles of coral roads were built in the area.

    Waterfront facilities were developed to provide minor fleet repairs. The Japanese pier was restored, and a 50-by-240-foot boat-slip was added to the outer end, forming an L. The depth alongside was 20 feet, and the pier could safely dock five LST’s or a large number of small craft at one time. The Seabees also constructed a log crib, coral-fill quay, 300 feet long, with two 3-by-12-pontoon arms, and established repair facilities for landing craft. A 250-ton pontoon floating drydock was assembled, and a 2,000-ton floating drydock, capable of handling destroyer escorts, was provided for the base. Half the 15th Special Battalion was divided between Kwajalein and Roi-Namur for stevedoring.

    Personnel were housed in floored and framed tents and in wooden frame barracks. Other base installations included the 200-bed 22nd Station Hospital, 80,000 square feet of covered storage, and a 12,000-barrel aviation-gasoline tank farm.

    Between June and September 1944, the two battalions sent detachments to erect a large fuel-oil tank farm on Bigej Island, north of Kwajalein Island. This consisted of four 50,000-barrel tanks and fourteen 10,000-barrel tank, with all appurtenances.

    CBMU 607 arrived in the area in August 1944, to relieve the construction battalions and take over maintenance and minor construction.

 Ebeye Island.—On March 7, 1944, four officers and 242 men of the 107th Battalion were sent to Ebeye Island to develop the seaplane base. As the two Japanese seaplane ramps had sustained only minor damage before their capture, construction work necessary for the development of the base, with the exception of paving the parking area and erecting ships and housing, was small.

    In April 1944, the atoll commander was instructed to move his headquarters from Kwajalein Island to Ebeye. A detachment of the 74th Battalion was sent there to handle the construction necessary to the development of the headquarters , pending completion of the seaplane base by the 107th.

    A Japanese pier, 1600 by 30 feet, with a 50-by-240-foot L extension, was repaired by the Seabees and a 250-foot Japanese H-shaped pier was also used. In addition, the Seabees assembled a pontoon wharf and pontoon barges for transporting damaged carrier aircraft to repair units ashore.

    Further installations on Ebeye consisted of Housing in floored tents and Quonset hut, a 150-bed dispensary, four magazines, 24,000 square feet of covered storage, and a 4,000-barrel aviation-gasoline tank farm.

    Although the war moved far forward, no roll-up was contemplated at Kwajalein, as it was necessary to maintain aerial surveillance of the bypassed Japanese bases in the Marshalls and Carolines.