Geography

Kwajalein Atoll is in the heart of the Marshall Islands. It lies in the Ralik Chain, 2,100 nmi (2,400 mi; 3,900 km) southwest of Honolulu, Hawaii at 8°43′N 167°44′E. Kwajalein is the world's largest coral atoll and comprises 93 islands and islets; it has a land area of 1,560 acres (6.33 km²)^[1]: 12 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 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.^[1]: 7 These islands had been Imperial German colonies, after their purchase from Spain in 1899.^[1]: 11 At the end of World War I they were assigned to Japan in the post-war settlement through the South Seas Mandate. The islands then became a mystery because the Japanese closed them to the outside world. It was presumed the Japanese had built illegal fortifications throughout the islands, but the extent of such fortifications was unknown.^[1]: 9 Japan regarded them as part of the "outer ring" of their territory.^[1]: 10

The strategic importance of the Marshalls had been recognized as early as 1921 in War Plan Orange, an American interwar plan for a possible conflict with Japan. The Marshalls were a key step in the island-hopping march to the Japanese mainland.^[1]: 7

After losing the Solomon Islands and New Guinea to the Allies in 1943, the Japanese command decided that the Gilbert and Marshall Islands were expendable.^[1]: 21 They preferred fighting a decisive battle closer to home. Nevertheless, the Marshalls were reinforced at the end of 1943 to make their capture more costly for the Americans. By January 1944, the regional commander in Truk, Admiral Masashi Kobayashi, had 28,000 troops to defend the Marshalls, although he had very few aircraft.^[1]: 33

Japanese planning and preparation

The 6th Base Force, under the command of Rear Admiral Monzo Akiyama, and headquartered on Kwajalein since August 1941, was the principal defense force of the islands.^[1]: 31 Akiyama had his men spread out over a very wide area, with naval-air bases located on Roi-Namur, Mili, Maloelap, Enewetak, and Wotje.^[1]: 31 Garrison troops on the island included the 1st Company, 3rd Mobile Battalion, 1st Amphibious Brigade, plus units of the 2nd Mobile Battalion.^[1]: 31

The defense system on the islands was mostly in line, with little or no depth.^[1]: 33 The Japanese had twin 12.7 cm guns on each end of the island plus 80mm guns on the ocean and lagoon sides.^[1]: 35 The 22nd Air Flotilla, depleted after the Gilbert campaign, had 128 aircraft in the Marshalls, 10 on Kwajalein.^[1]: 31

US planning and preparation

The Marshall campaign planned by the US involved attacks on seven islands, bypassing many more.^[1]: 18 Operation Flintlock had nine phases, the main phase being the capture of Kwajalein and Majuro Atolls, a second phase, Operation Catchpole, being the capture of Eniwetok, and further phases being the capture of remaining islands.^[1]: 18 Bombardment by the Seventh Air Force, and a carrier air attack on 4 December 1943, followed by additional attacks in January 1944, destroyed all Japanese aircraft.^{[1]: 36, 18}

Staging through Baker Island airfield, Consolidated B-24 Liberators bombed strategic targets. In the beginning, the most important was Mili, the Japanese base closest to the Gilberts and Maloelap, the most powerful enemy bases threatening the upcoming operations. Mili was the subject of several attacks throughout November, causing considerable damage to installations and high losses of aircraft for the Japanese. But Mili 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 122 short tons (111 t) of explosives on Mili'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 Mili 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 North American B-25 Mitchells and Curtiss P-40 Warhawks.

It was necessary to take another atoll in the eastern Marshalls—Majuro. This feature is 220 mi (190 nmi; 350 km) 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 the V Amphibious Corps Marine Reconnaissance Company and the 2nd Battalion, 106th Infantry, 27th Infantry Division were employed in its capture. The island was taken on 31 January 1944 without any US casualties.^{[1]: 37–38}