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Operation Flintlock (31 January-4 February 1944) was the first part of the American invasion of the Marshalls, and saw them conquer Kwajalein Atoll and Majuro, giving them a foothold in the central part of the islands.
The Marshalls had been an American target since the start of the Pacific War. They were the most vulnerable pre-war Japanese territory, and their invasion would allow the Americans to threaten the inner ring of Japanese defences. There was also a hope that they might bring the Japanese fleet out for a decisive battle. Two targets were selected for the first stage of the invasion - Kwajalein Atoll in the centre of the group and Majuro at the eastern end. Within Kwajalein Atoll there were three main targets - Kwajalein island in the south and Roi and Namur in the north.
Overall command of the operation was given to Admiral Spruance, as commander of the Fifth Fleet (Task Force 50). Admiral Turner commanded the Joint Expeditionary Force (Task Force 50), with responsibility for the naval aspects of the plan, and a veto over any major changes to the landing plans. General Holland Smith commanded the troops involved in the invasion (Task Force 56).
Three naval attack forces and three landing forces were also created. The naval attack forces came under Admiral Turner and the landing forces under General Smith. The naval commander would be in charge until the beach head was secure enough for the land commander to move ashore.
Kwajalein was to be invaded by the Southern Attack Force (Task Force 52), also under Admiral Turner and the Southern Landing Force (Task Group 56.1), made up of the 7th Infantry Division (Major General Charles H. Corlett).
Roi and Namur were to be attacked by the Northern Attack Force (Task Force 53) under Rear Admiral Richard L. Conolly and the Northern Landing Force, made up of the 4th Marine Division (Major General Harry Schmidt).
Majuro was to be attacked by the Majuro Attack Group (Task Group 51.2) under Admiral Hill and the Majuro Landing Force, mad up of the 2nd Battalion, 106th Infantry under Lt. Col Frederick B. Sheldon.
The three naval attack groups were powerful fleets, containing the older battleships that became specialists in pre-invasion bombardments, cruisers, destroyer and escort carriers. They also included two dedicated command ships USS Rocky Mount and USS Appalachian and the partly modified HQ ship APA Cambria.
The invasion was also supported by two new weapons - the LVT(A), an armoured amphibian tractor with a 37mm gun in a turret and the LCI(G) gunboat, an landing craft armed with three 40mm guns and batteries of 4.5in rockets. Both were to be used to provide close support for the actual landings.
The invasion would be supported by three other commands. The most potent was the Carrier Force (Task Force 58), under Rear Admiral Marc. A. Mitscher (more famous as the Fast Carrier Task Force). This consisted of four task groups of fast carriers, each supported by fast battleships, cruisers and destroyers. Second was the Neutralization Group (Task Group 50.15) contained three heavy cruisers, four destroyers and two mine sweepers (Admiral Ernest G. Small). Third was the Defense Forces and Land Based Air (Task Force 57) under Admiral Hoover, which covered all of the land based aircraft
Admiral Hoover's force would begin the operation, attacking the Japanese airfields in the islands and dropping mines around the islands that were being bypassed.
The fast carrier task force would be used to destroy Japanese air facilities on Wotje, Maloelap, Roi-Namur and Kwajalein on D-2, then bombard Wotje and Maloelap on D-1, supported by part of the invasion force and the neutralization force. On D-Day itself its role would be to provide air support over Kwajalein.
The neutralization force's main role was to conduct a naval bombardment of Wotje and Maloelap from 29 January onwards.
After a careful examination of Kwajalein Island, the Americans decided to land at its western tip, where the defences appeared to be weakest. The invasion of Kwajalein would take place on D+1 (1 February 1944). On D-Day itself the Americans were planning to capture the next four islands to the west - Carlson, Carlos, Carter and Cecil Islands. Carlson was to be used as an artillery base, the other three islands were needed to secure Cecil Pass, the best deep water route into the southern part of the Atoll, which ran between Carter and Cecil Islands.
Roi and Namur would also be invaded on D+1, this time landing on the southern (lagoon) side. On D-Day itself Jacob and Ivan islands to the west of Roi and Albert and Allen to the south-east of Namur would be occupied, ready to be used as artillery bases.
At Majuro the V Amphibious Corps Reconnaissance Company would occupy Calalin Island, in the entrance to the Atoll, and the main invasion would then depend on what they found on Majuro itself.
After a month long preliminary air attack by the Seventh Air Force, most of the Japanese airfields had been neutralized. Only Rou-Namur still posed a threat, mainly because it was furthest away, and thus a more difficult target.
The main battle began on 29 January (D-2), when Task Force 58 (the fast carriers) entered the Marshalls and began to attack the remaining Japanese defences. Each of the four task groups had a different target. TG 58.1 (Enterprise, Yorktown and Belleau Wood) attacked Taroa, the largest island at Maloelap Atoll, and neutralised the dangerous airfield there. TG 58.2 (Essex, Intrepid and Cabot) attacked Roi-Namur, where the Japanese still had 92 aircraft. The carrier attack quickly eliminated the threat, and no Japanese aircraft were in the air after 0800. TG 58.3 (Cowpens, Monterey and Bunker Hill) attacked Kwajalein Island itself then moved north-west ready to attack Eniwetok on the following day. TG 58.4 (Saratoga, Princeton and Langley) attacked Wotje. The carrier attacks were supported by more land based aircraft.
On 30 January TG 58.1 moved on to Kwajalein. TG 58.2 remained focused on Roi-Namur. TG 58.3 attacked Eniwetok. TG 58.4 attacked Wotje and Taroa.
Kwajalein Island and the southern Atoll
D-Day for the invasion was 31 January 1944, with the occupation of the smaller islands. Carlson, Carlos, Carter and Cecil Islands were all occupied in a single day, and a foothold was accidently created on Chauncey Island (later completely occupied on 2 February).
The invasion of Kwajalein Island began on 1 February 1944. The initial landings went well, and despite some bursts of determined resistance, the Americans were able to advance steadily across the island. About half of the defenders had probably been killed in the pre-invasion bombardment, and the rest were unable to put up a terribly organised fight. By the end of 4 February the Americans had reached the north-eastern corner of the island, and it was declared to have been secured.
While Kwajalein was still being secured, attention turned to Burton Island, the next major island in line to the north. This was captured on 3-4 February. Buster and Byron Islands, two tiny reefs to the south of Burton Island, were occupied on 3 February. On 4 February attention turned to Burnet and Blakenship, to the north. Burnet was undefended, but around 20 Japanese sailors and Korean labourers, marooned on the island, had to be overcome on Blakenship.
The 2nd Battalion, 17th Infantry, was then split into Eastern and Western Forces, to complete the occupation of the lines of islands on either side of the southern part of the Atoll. The Eastern Force met no opposition and secured five islands. The Western Force was less lucky, running into a small Japanese force on Clifton Island on 5 February, which was overcome at the cost of 1 dead and 4 wounded.
The 17th Infantry was given the task of clearing the islands on the south-eastern corner of the atoll. On 5 February they overcame limited resistance on Beverly Island, Berlin Island (where 198 Japanese were killed and one captured) and Benson Island. The 7th Reconnaissance Troop with the help of the 184th Infantry captured Bennett Island, having run into more resistance than on the other smaller islands.
Roi, Namur and the Northern Atoll
D-Day for Roi and Namur was also 31 January. Once again a series of smaller islands were invaded on this day, although here a rather over-complex plan involving re-using a limited number of LVTs didn't go well. Even so Jacob, Ivan, Albert, Allen and Abraham Islands were all captured during the day.
Roi and Namur were both invaded on 1 February. Roi, which was largely dominated by a Japanese airfield, fell in a single day. Namur, where there was still plenty of undergrowth, held out a little longer, but only until 2 February. Between them these two attacks cost the Americans 190 dead and 547 wounded, while the Japanese lost 3,500 dead and 264 captured
Majuro
The occupation of Majuro Atoll was the easiest. It wasn't clear how well the islands would be defended, and so plans were in place to deal with fairly sizable garrisons. However it quickly became clear that there were few, if any, Japanese troops on the island, and eventually only a single naval officer was found, effectively acting as a caretaker!
Aftermath
The easy conquest of Kwajalein and Majuro meant that the Americans were able to bring forward the invasion of Eniwetok Atoll from May to mid-February (Operation Catchpole). As a result the newly arrived Japanese defenders of that atoll were unable to prepare proper defences, and the main islands fell in only six days. The Americans had soon taken near total control of the Marshalls, which became a major air and naval base for the invasion of the Mariana Islands. Only Wotje, Mille, Jaluit and Maloelap remained in Japanese hands and they were easily neutralised and allowed to wither on the vine. The outer line of Japanese defences had been penetrated.