Battle of Kwajalein The Battle of Kwajalein was fought as part of the Pacific campaign of World War II . It took place from 31 January - 3 February 1944 , on Kwajalein Atoll in the Marshall Islands . Employing the hard - learned lessons of the battle of Tarawa , the United States launched a successful twin assault on the main islands of Kwajalein in the south and Roi - Namur in the north . The Japanese defenders put up stiff resistance , although outnumbered and under - prepared . The determined defense of Roi - Namur left only 51 survivors of an original garrison of 3 , 500 . For the US , the battle represented both the next step in its island - hopping march to Japan and a significant moral victory because it was the first time the Americans had penetrated the " outer ring " of the Japanese Pacific sphere . For the Japanese , the battle represented the failure of the beach - line defense . Japanese defenses became prepared in depth , and the battles of Peleliu , Guam , and the Marianas proved far more costly to the US . Background . Geography . Kwajalein Atoll is in the heart of the Marshall Islands . It lies in the Ralik Chain , southwest of Honolulu , Hawaii at . Kwajalein is the world ' s largest coral atoll as measured by the area of enclosed water . Comprising 97 islets , it has a land area of 1 , 560 acres ( 6 . 33 km² ) and surrounds one of the largest lagoons in the world , measuring 324 mi² ( 839 km² ) in size . The two most significant land masses are Kwajalein Island in the south , and the linked islands of Roi - Namur in the north . By the start of World War II , the Marshalls ( South Pacific Mandate ) were already an integral part of the Japanese perimeter of defense . Its facilities were being utilized as outlying bases for submarines and surface warships , as well as for air staging for future advances being planned against Ellice , the Fiji Islands , and Samoa . Gilbert and Marshall Islands campaign . After the capture of Makin and Tarawa in the Gilbert Islands , the next step in the United States Navy ' s campaign in the central Pacific was the Marshall Island chain . These islands had been German colonies until World War I , when they were assigned to Japan in the post - war settlement as the " Eastern Mandates " . The islands had become a mystery since the Japanese closed them to the outside world . It was presumed the Japanese had built illegal fortifications throughout the islands , however the precise extent of any such fortifications was unknown . To the Japanese , regarding them as part of the " outer ring " of Japanese territory held prior to 1941 , this assault would be the first on what the Japanese regarded as Japanese soil . The strategic importance of the Marshalls had been identified as early as 1921 in Plan Orange — the American interwar plan for a possible conflict with Japan . The Marshalls were a key step in the island - hopping march to mainland Japan . After the loss of the Solomon Islands and New Guinea to the Allies in 1943 , the Japanese command decided that the Gilbert and Marshall Islands would be expendable . They preferred to fight a decisive battle closer to home . However , at the end of 1943 , the Marshalls were reinforced to make their taking expensive for the Americans . By January 1944 , the regional commander in Truk — Admiral Masashi Kobayashi — had 28 , 000 troops to defend the Marshalls , but he had very few planes . Japanese planning and preparations . Actual defenses on the Marshalls , however , were never very substantial or heavily manned . After nearly 10 years of construction , fortifications on the Marshalls were considerably inferior when compared with Tarawa , which had been turned into a fortress in less than eighteen months . Furthermore , less than half of the troops stationed in the Marshalls were combat trained , the rest being support and labor troops with little or no combat training . It was not until after the Japanese position in the Solomons and New Guinea began to deteriorate that Imperial Headquarters made plans to strengthen the Marshalls . In fact , by September 1943 , the Japanese High Command had written off the Gilberts and Marshalls as lost , deciding that the areas should be used to fight a delaying action while a new defense perimeter was created from the Banda Sea through Caroline Island and the Marianas Islands . Additionally , combat units were ordered to the Marshalls from the Philippines , Manchukuo , and the homeland , with additional air power to be flown in from both the homeland and nearby Truk . The 6th Base Force , under the command of Rear Admiral Monzo Akiyama , and headquartered on Kwajalein , was the principal defense force of the islands . Akiyama , however , had his men spread out over a very wide area , mostly concentrating on the defense of those atolls ( Jaluit , Mille , Maloelap , and Wotje ) , that were never considered vulnerable to American attack . Those reinforcement troops included 2nd Battalion / 1st Amphibious Brigade under Captain ( Naval ) Aso , nine Type 94 / 97 Tankettes and two companies of Type 2 Amphibious Tank ( in the Japanese Special Naval Landing Forces ( SNLF ) ) , or Kwajalein Tank Detachment ) . They finally arrived and were quickly dispersed by Akiyama , mainly to the outlying atolls . Kwajalein was to remain undermanned , underequipped , and unprepared for the assaults being arrayed against it . Overall Japanese strength on these islands numbered approximately 8 , 000 men , of which less than half were considered combat effective . On Kwajalein proper , the troops were made up mostly of labor forces , a good number of those being Koreans . On Roi - Namur , the troops were mostly JNAF land personnel who had little ground combat training and who were underequipped for such a function . The defense system on the islands was mostly in line , with little or no depth . Although some fortified areas existed , none were as extensive or well armed as those of Tarawa . In addition , there were several defenses that mostly concentrated on any assault coming from the ocean side ; no coastal defense artillery had been placed on key islets guarding passages to the lagoon , and there was little or no use of mines . Despite shortfalls , efforts to strengthen ground defenses continued . Still , Akiyama ' s greatest defense remained in his aerial offensive capability . He had well - manned air bases on Roi - Namur , Maloelap , Wotje , Mille and Eniwetok , detached Chitose and 653rd Air Corps Mitsubishi A6M Zero fighters with the nearly complete Mitsubishi G3M and G4M bomber base in Kwajalein . In addition , he had some Nakajima A6M2 - N and Mitsubishi F1M floatplane s detached in seaplane bases on Jaluit , Wotje , Majuro , Taongi , Utirik , and the south Kwajalein Atoll island of Burton . During the month of November , however , both USAAF land - based and US Navy carrier - based bomber attacks , in conjunction with the American assault on the Gilberts , had destroyed 71 of Akiyama ' s fighters and bombers . Reinforcements flown in from the homeland and Truk replenished most of his losses , but he could expect little additional help in the future . The Japanese war industry was falling far short of needed production , affecting all branches of the armed services , including the Japanese Carrier Air Arm , which had long since retreated from the Central Pacific . In fact , the 32 planes flown from Truk were the last of the carrier aircraft left behind following that retreat . Akiyama was therefore not expected to defeat his adversary , but rather to delay the Allied forces advance while exacting the greatest possible toll upon them . US planning and preparation . Expecting the US to attack the outermost islands in the group first , most of the defenders were stationed on the atolls of Wotje , Mille , Maloelap , and Jaluit to the east and south . Indeed , the original American plan called for a cautious series of steppingstone attacks starting in the eastern Marshalls . The Japanese troop dispositions were revealed to the Americans by Ultra decryptions of Japanese communications ; Admiral Chester Nimitz , commander of American naval forces in the Pacific , decided to bypass these outposts and attack the Kwajalein Atoll directly , knifing into the very heart of the Marshall Islands . The operation was given the code name Operation " Flintlock " . There were two main objectives : The linked islands of Roi - Namur in the north , and Kwajalein Island at the southern end of the atoll . The 4th Marine Division — under Major General Harry Schmidt — was assigned to Roi - Namur , and the Army ' s 7th Infantry Division — under Major General Charles H . Corlett — would make the assault on Kwajalein . To ensure the success of the operation , sea and air superiority were necessary . Accordingly , on 29 January 1944 , US carrier planes attacked the Japanese airfield on Roi - Namur , destroying 92 of the 110 Japanese planes in the Marshalls . Staging through Baker Island airfield , B - 24 Liberator s of the Seventh Air Force quickly set their sights upon their targets . In the beginning , the most important was Mili Atoll , the Japanese base closest to the Gilberts and Maloelap , the most powerful enemy bases threatening the upcoming operations . Mille was the subject of several attacks throughout November , causing considerable damage to installations and high losses of aircraft for the Japanese . But Mille remained the only base within fighter reach of the Gilberts , and the defenders managed to keep the facilities there operational and reinforced with aircraft . Following the capture of Tarawa and until 19 December , 106 B - 24s dropped a total of of explosives on Mille ' s airbase . The largest of those raids came on 4 December when 34 B - 24s pulverized the atoll in conjunction with carrier - based bombing raids of other parts of the Marshalls . On 18 December , renewed strikes were initiated against enemy targets on Mille with land - based Douglas A - 24 Banshee dive bombers and Bell P - 39 Airacobra fighters making their debut in the Marshall air offensive . Japanese losses for the day amounted to 10 fighters ( four on the ground ) and four damaged . Other aircraft types participating in the offensive included B - 25 Mitchell s and Curtiss P - 40 Warhawk s . In addition , it was necessary to take another atoll in the eastern Marshalls — Majuro . This feature is southeast of Kwajalein and could serve as an advanced air and naval base as well as safeguard supply lines to Kwajalein . Majuro was very lightly defended and only the V Amphibious Corps Marine Reconnaissance Company and the 2nd Battalion , 106th Infantry , 7th Infantry Division were employed in its capture . The island was taken on 31 January 1944 , without any US casualties . Battle . The US forces for the landings were Rear Admiral Richmond K . Turner ' s 5th Amphibious Force , and Major General Holland M . Smith ' s V Amphibious Corps , which comprised the 4th Marine Division commanded by Maj . Gen . Harry Schmidt , the 7th Infantry Division commanded by Maj . Gen . Charles H . Corlett , plus the 22nd Marines , 106th Infantry , and the 111th Infantry regiments . The 4th and 7th Divisions were assigned to the initial landings at Kwajalein , while the 2nd Battalion of the 106th was assigned to the simultaneous capture of Majuro Atoll . The rest of the 106th and the 22nd Marines were in reserve for Kwajalein , while awaiting the following assault on Eniwetok , scheduled for three months later . The 7th Infantry Division began by capturing the small islands labeled Carlos , Carter , Cecil , and Carlson on 31 January , which were used as artillery bases for the next day ' s assault . Kwajalein Island is long but only wide . There was therefore no possibility of defence in depth , so the Japanese planned to counter - attack the landing beaches . They had not realized until the battle of Tarawa that American amphibious vehicles could cross coral reefs and so land on the lagoon side of an atoll ; accordingly the strongest defences on Kwajalein faced the ocean . The bombardment by battleship s , B - 24 bombers from Apamama and artillery on Carlson was devastating . The US Army history of the battle quotes a participant as saying that " the entire island looked as if it had been picked up 20 , 000 feet and then dropped . " By the time the 7th Division landed on Kwajalein Island on 1 February , there was little resistance ; by night the Americans estimated that only 1 , 500 of the original 5 , 000 defenders were still alive . On the north side of the atoll , the 4th Marine Division followed the same plan , first capturing islets Ivan , Jacob , Albert , Allen , and Abraham on 31 January , and landing on Roi - Namur on 1 February . The airfield on Roi ( the eastern half ) , was captured quickly , and Namur ( the western half ) , fell the next day . The worst setback came when a Marine demolition team threw a satchel charge of high explosives into a Japanese bunker which turned out to be a torpedo warhead magazine . The resulting explosion killed twenty Marines and wounded dozens more . Only 51 of the original 3 , 500 Japanese defenders of Roi - Namur survived to be captured . Aftermath . The relatively easy capture of Kwajalein demonstrated US amphibious capabilities and showed that the changes to training and tactics after the bloody battle of Tarawa had been effective . It allowed Nimitz to speed up operations in the Marshalls and invade Eniwetok Atoll on 17 February 1944 . The Japanese learned from the battle that beachline defenses were too vulnerable to bombardment by ships and planes . In the campaign for the Mariana Islands , the defense in depth on Guam and Peleliu was much harder to overcome than the thin line on Kwajalein .