The first week of February 2024 marked the 80th Anniversary of Operation FLINTLOCK, the U.S. invasion of the Marshall Islands during WWII. This operation, the largest amphibious assault of the war to date, was the first major central Pacific operation of the Pacific War. The successful landings in the Marshalls, both at Roi-Namur and Kwajalein, demonstrated the soundness of American amphibious operation doctrine, and revealed that the issues of poor planning and execution seen at Guadalcanal, the Aleutians and Gilberts were resolved. The complex operation was swiftly and efficiently executed, and the vast swath of islands in the central Pacific were consolidated for subsequent operations against the Japanese.
The operation was only possible owing to the vast expansion in the size and capacity of the U.S. Navy since the attack on Pearl Harbor. From the mere 79 ships that took part in the invasion of Guadalcanal in August 1942, and those scraped together from every port in the Pacific, over 300 ships took part in Flintlock, with these representing a small portion of the entire USN. The Task Forces for Flintlock included four three-carrier Task Forces (carrying 700+ aircraft), along with three escort carriers, supporting battleships, cruisers, and destroyers as well as hundreds of cargo ships, attack transports, oilers, tank landing ships, and specialized landing craft. Planning for an attack on this scale, where every man, bullet, and gallon of gas had to be transported hundreds (or even thousands) of miles, [1] had to start far in advance of D-Day. This included intelligence analysis, to determine enemy strength and thus friendly forces required, as well as planning a scheme of maneuver, ship-to-shore assault, and naval gunfire and air support to the landing forces. Detailed communications and logistics plans were also crucial. Assault forces were identified – the 7th Infantry Division for Kwajalein and 4th Marine Division for Roi-Namur – and trained. Of particular importance for the Marshall Islands were amphibious tractors (called LVTs), which were needed to cross the fringing coral reefs or shelfs surrounding the atolls. As the tractors were not heavily armored, the landing force was also supported by amphibious tank companies to help drive the attack inland.
Although pre-invasion air and naval attacks to attrition Japanese forces in the Marshalls began as early as November 1943, Japanese air and naval forces were not completely neutralized until a series of carrier air strikes wiped out the last enemy aircraft in the region on 29 January 1944. The actual invasion started with D-Day on 31 January, when 7th Infantry Division Soldiers landed on Cecil and Carter Islands, just northwest of Kwajalein, and 4th Division 25th Marines landed on Jacob and Ivan islands southwest of Roi-Namur. Both toe-hold landings expanded later that day, taking additional nearby islands to establish artillery fire support bases. By that night, five Army artillery battalions were registered on Carlson Island and four Marine artillery battalions on Abraham and Albert islands, both designed to provide quick, accurate and responsive fire support to cover the landing beaches at Kwajalein and Roi-Namur. As Abraham was only 460 yards from Namur, Marines also lugged anti-aircraft guns, mortars, and heavy machine guns ashore to provide direct fire support for the main landings scheduled the next day. The outlying islands also secured the few lagoon inlets, important deep water passages into the calm interior waters of the atoll. From there, landing craft could more easily form up for the main landings and rapidly offload combat troops, tanks, supplies, and equipment following the main assaults.
All day on the 31st, as well as the morning of D+1, 1 February, a half dozen battleships and cruisers supported by a dozen more destroyers fired over 15,000 rounds of high explosive and armor-piercing shells at Japanese bunkers and other strongpoints on or near the invasion beaches. Carrier aviation and land-based Army Air Corps bombers dropped several tons of bombs on hard to spot targets. One observer described the bombardment of Kwajalein as "devastating-the entire island looked as if it had been picked up 20,000 feet and then dropped-All beach defenses were completely destroyed, including medium and heavy anti-aircraft batteries." At Roi-Namur, in sharp contrast to pre-war conceptions of shore bombardment, Navy warships closed to within 3,500 yards of the beach to conduct point-blank attacks on bunkers and pill boxes. The Marines were so impressed with battleships firing at less than a mile offshore that they nicknamed the Admiral “Close-in Connelly.” According to Samuel Elliott Morison, even Marine Gen. Smith, Commander V Corps, “loved the Navy after this assault – for a few days at least.”
Although reveille had sounded at 0130 for the Marines, and the attack was planned for a conservative 1000, the landings were delayed by a shortage of LVTs. Still, last minute support bombardments went perfectly, preventing the Japanese from taking advantage of the delays. It was a synchronized crescendo of artillery from nearby islands, rocket fire from LCIs, gunfire from heavy ships, and air strikes from carriers all covering the landing craft during their 33-minute run to the beach. At exactly 500 yards offshore an air observer dropped a parachute flare and long-distance support fire stopped like clockwork. Amphibious tanks and the LVTs then took up the task of suppressing the beaches. In fact, the repeated bombardments had so decimated Japanese defenses on Roi that the “first two waves landed and advanced standing up.”
Indeed, to the Marines, Roi appeared completely devastated. The island was mostly airfield, so the open spaces were quickly occupied as 1st and 2nd battalions of 23rd Marines moved inland, supported by two companies of tanks. Following a delay caused by communication problems, the Marines re-organized and cleared the remaining Japanese bunkers and pillboxes that afternoon and the island was declared secure at 1735.
In contrast, 24th Marines at Namur ran into trouble from the start. In addition to armored vehicles getting stuck in an unforeseen anti-tank ditch, the vast amounts of wreckage led to beach congestion as follow on companies landed on top of each other at irregular intervals. In response, Marines began pushing inland on foot, reducing pill boxes and fortifications with satchel charges and bazookas as they advanced. During one clearing operation, a team from Fox Company, 1st Battalion, 24th Marines, tossed a satchel charge into a large blockhouse. Suddenly, the largest ammunition bunker in the Marshall Islands, full of torpedoes and 500 kg bombs, detonated in a gigantic explosion. The shock wave buffeted planes in the air and flung debris a thousand feet – logs, chunks of concrete, steel, and pieces of bodies rained down over a huge swathe of the island. Some 20 Marines were killed, including some still in boats in the lagoon, and over 100 wounded.
The explosion completely disrupted the attack and set everything to confusion. It took several hours to reorganize the front lines and bring in reinforcements. By that time, a line was stabilized across the middle of the island, ending in front of the Japanese blockhouse and fortifications at Nadine Point, which had held out all day against ferocious fire attacks from guns and mortars on Abraham Island. That night, Japanese infiltrators plagued the line of contact, throwing grenades and jumping into foxholes to attack with swords and bayonets. Marines responded with their own grenades, mortars, and sniping. These attacks eventually helped define Marine rules for night combat in the Pacific: don’t startle your buddies, don’t shoot unless absolutely necessary, and above all don’t leave your hole. The ultimate rule was shoot first and seek forgiveness in the morning.
Just before dawn on the second morning, about 100 Japanese conducted a series of counter-attacks that were destroyed after desperate hand-to-hand fighting. At roughly the same time, 24th Marines ordered a two battalion pincer attack intending to split Japanese defenses and meet at Natalie Point, the northern most beach on Roi. The attack quickly broke Japanese resistance, and the Marines pushed forward, blasting pillboxes, throwing grenades, chasing enemy soldiers out of trenches. The last organized resistance was destroyed around 1215 that day, though the fighting did not end. The ground had to be re-checked, holes inspected, and bunkers entered. Occasional shouts of surprise, bursts of gunfire, and grenade blasts marked the rest of the day. Marines joked the Japanese were the last to get the word they had lost.
The islands were secured at a cost of 194 dead and 436 wounded. Of the defending force of 4,000, only 130 Japanese and 167 Korean laborers survived. That night thousands of men slept in the company of thousands of the dead.
At Kwajalein, the Army’s 7th Division faced a tough battle over a much larger island. There too, the landing force comprised two regiments landing abreast. The 184th on the left, and 32nd on the right. Despite heavy pre-landing bombardments, the Japanese engaged immediately with rifle and machine gun fire. Several companies of tanks came ashore within the first hour, and the two regiments began to slowly and methodically reduce Japanese strongholds. The airfield was taken by the end of the second day, the administrative buildings (reduced to rubble by bombardments) cleared by the third, and the last Japanese cleared out by 1030 on 4 February.
Kwajalein was secured at a cost of 177 killed and 1,000 wounded. Of the defending force of 5,000, only 49 Japanese and 125 Korean laborers survived. Analysis suggested that between 50 to 75 percent of the Japanese were killed in the preparatory bombardments.
As the fighting ended, all three islands were consolidated, with Seabees and Army engineers immediately repairing airfields and establishing supply and fuel depots. Days later, American planes started attacking the remaining Marshall islands still under Japanese control. Within weeks, bombers from the Gilberts had staged to the Marshalls and were conducting raids on the Japanese naval base at Truk.
The conquest of the Marshall Islands demonstrated the soundness of American amphibious doctrine, albeit on a smaller scale than practiced later in the war. As with earlier operations, American planners acquired much experience in the campaign and applied new techniques which would be shaped and refined for subsequent operations. Two of the most important lessons involved close air support and naval gunfire. Before 1944 continuous support from those services was more the exception than the rule. With the destruction of Japanese naval and air forces as the war progressed, and given American industry's tremendous output in ships and planes, the possibility of greatly increasing air and naval pre-invasion fire support became both feasible and desirable.
The quick seizure of the Marshall Islands allowed Admiral Nimitz to advance the date for the invasion of the Marianas by almost six months. At the same time, Japanese preparation time for the defense of islands like Saipan and Tinian was shortened, and many of the U.S. units that had participated in the Marshalls operation, still relatively intact, were made available. The overall importance of the rapid seizure of the Eastern Mandates thus cannot be overestimated. The Marshalls were not only a proving ground for new tactics and innovations but also a critical step in a chain of events that led to the Japanese surrender at Tokyo Bay.
[1] If a land army’s logistics is akin to a centipede (eating as it marches), naval logistics, especially on an overseas or global scale, is more akin to a python, having to swallow and carry all its sustenance in one go.