The Marianas Campaign (14 June-10 August 1944) was a key stage in the Pacific War, triggering the battle of the Philippine Sea at which the Japanese naval aviation forces were almost destroyed, and bringing Japan within range of B-29 bombers based on the islands.
On 12 March 1944 the US Joint Chiefs of Staff issued a directive to Admiral Nimitz to prepare for an invasion of the Mariana Islands, starting on 15 June 1944. The aim was to isolate the Japanese bases in the Caroline Islands, and gain air and sea bases for attacks on Japanese sea lanes and the Home Islands. This was the end result of a long series of debates over the correct Allied strategy in the Pacific, in which Admiral King was an advocate of an invasion of the Marianas at a time when the assumption was that there would be a direct attack on the Japanese bases in the Caroline Islands. He was opposed by MacArthur, who saw it as a distraction from his plans for a return to the Philippines, but supported by the USAAF, which wanted the islands as a base for the new B-29 Superfortress, which was expected to enter service during 1944.
King finally got his way at the Sextant conference in Cairo in December 1943, when the invasion of Guam and the Japanese Mariana Islands was officially approved, and set for 1 October 1944, in the aftermath of the planned invasions of the Marshalls, Ponape and Truk. The invasion of the Marshalls took less time than had been expected, and early in 1944 the invasion of Truk was cancelled. This allowed the invasion of the Marianas to be brought forward. Truk was to be neutralized, and Nimitz was then to invade the Marianas on 15 June and the Palaus on 15 September, while MacArthur continued his advance towards the Philippines.
The Marianas had been a Spanish possession until the late 19th century, when their control was challenged by Germany and the United States. In 1899 Spain sold all of her possessions in the Carolines, Marshalls and Marianas to the German Empire, including Saipan and Tinian. At about the same time Guam was conquered by the United States during the Spanish-American War. The German presence was short-lived - they lost the islands to the Japanese in the first year of the First World War, and their conquests were confirmed in the peace treaties at the end of the war. When the Japanese entered the Second World War they conquered Guam on 10 December 1941, after a three hour battle at the capital.
The Marianas were an important Japanese base by the summer of 1944, the base of the Japanese Central Pacific Fleet under Admiral Nagumo Chuichi and the 31st Army under General Obata Hideyoshi. The islands were within the Japanese outer defences, and so their conquest would mark the clear failure of the Japanese defensive plan.
There were three main islands in the southern Marianas - Guam in the south, Saipan in the north and Tinian just to the south of Saipan (the last two were within artillery range of each other, Guam about a hundred miles to the south). The American plan was to invade Saipan first, and then attack Guam and Tinian at about the same time. Two amphibious corps would be involved in the battle - the 5th Corps under General Holland Smith and the newly formed 3rd Amphibious Corps under General Roy Gieger.
Planning for the operation began with a major reconnaissance effort. This began on 22-23 February with twenty five reconnaissance sorties over Saipan and Tinian, launched from Mitscher's fast carrier Task Force 58. On 18 April five Navy PB4Ys escorted by Army bombers flew from Emiwetok to photograph Saipan and Tinian, and on 25 April a similar mission was carried out over Guam. Six Navy Liberators photographed Guam on 7 May and all three islands were photographed again on 29 May.
The islands were also examined by the sea, by the submarine USS Greenling, which carried out a series of photographic sorties between 2 and 29 April, focuses on the approaches to the beaches.
Two corps were allocated to the campaign. The V Amphibious Corps, based on Hawaii, was to attack Saipan and Tinian. The III Amphibious Corps, based on Guadalcanal, would attack Guam.
The invasion of Saipan began on 15 June when two marine divisions landed side by side on the west coast - the 2nd Division (General Thomas Watson) on the left and the 4th Division (General Harry Schmidt) on the right. By the end of the first day the Americans were one mile inland. The 27th Infantry Division (General Ralph Smith) landed on 16 June, and on 17 June captured Aslito (Isely) Airfield. Six days later P-47 Thunderbolts began to operate from the airfield, an impressive example of the speed with which US combat engineers could work. One of the main stories of the rest of the battle was the rising level of friction between the Marines and the Army. This began with a rather petty dispute over who had authority over an army regiment that was left to clear up the south-eastern corner of the island, and developed into a more serious problem after the Americans began to move north on 23 June. The three divisions advanced in line, with the 2nd on the left, 27th in the middle and 4th on the right. The 27th ran into heavy resistance, and were forced back. General Holland Smith blamed General Ralph Smith, and insisted on removing him from command. He was temporarily replaced by General Sanderford Jarman, commander of the post-conquest garrison, and then by General George Griner. These changes didn’t really speed up the advance, but they did poison the relationship between the Army and the Marines. Despite this despite the Americans steadily pushed north, and on 6 July, with the battle lost, Nagumo and Saito committed suicide. Many of the surviving Japanese troops then launched a series of mass banzai attacks, losing 2,500 men to no effect. 9 July saw the famous mass suicide at Marpi Point at the northern end of the island, after which Saipan was considered to be secure.
The fighting on Saipan was briefly affected by a major Japanese naval offensive. The Japanese Navy had long been planning for the 'decisive battle' in which they would destroy the US fleet and turn the tide of the war. The invasion of the Marianas gave them their chance, but the plan went badly wrong. The idea was to launch a massive carrier attack from beyond the range of US carrier aircraft, and use the airfields on the Marianas to land the aircraft. Instead the long range flights meant that most of the Japanese aircraft failed to find their targets. The Americans controlled the skies over the islands, so most of the aircraft that survived the aerial battles over the sea were lost over the islands. The battle of the Philippine Sea (19-20 June 1944) was so one-sided that it became know as the Marianas Turkey Shoot. The great Japanese gamble had failed. The IJN was never able to rebuild its naval aviation forces, and the defenders of the islands were depressed by the failure of the great attack.
The second landing of the campaign was on Guam, the former American possession lost on 10 December 1941. The original plan had been to launch the invasion on 18 June, three days after the invasion of Saipan, but a combination of the battle of the Philippine Sea and the need to commit the only reserve for the invasion on Saipan instead meant that the invasion had to be postponed until 21 July. The Americans landed on beaches five miles apart, separated by the Orote Peninsula. It took four days for the two flanks to join up, by which time they had already repulsed two night-time Japanese counterattacks, on the night of 25-26 July that greatly weakened the Japanese. Once the two beachheads were united, the Americans were free to push north, and they reached the north coast of the island on 10 August, when Guam was declared to be secure.
The conquest of Tinian was probably the most successful American amphibious assault of the Pacific War. The most obvious landing area was at Tinian town in the south-west of the island, but when the invasion came on 24 July the main effort actually came in the north-west, where the 4th Marines landed supported by naval, air and artillery support (coming from nearby Saipan). The Japanese were caught entirely by surprise, and by the end of the day there were 15,000 US troops on the island and the Japanese were outnumbered. That night they launched a costly banzai attack on the US lines, losing over 1,200 dead with no effect. This greatly weakened any chances they had of a long defence. Over the next few days the 2nd and 4th Marines forced their way down the island. On 31 July they attacked the last Japanese defensive positions, at the top of an escarpment at the southern end of the island, and on 1 August Tinian was declared to be secure.
Two other islands in the group, Rota and Pagan, both still had small Japanese garrisons, but they were left to sit out the war, subjected to occasional air raids to keep them down. Both finally surrendered on 2 September 1945.
The fall of Saipan caused a great crisis in Japan. The ease with which the Americans had penetrated the inner line of island defences, and the great naval defeat at the Philippine Sea, led Prime Minister Tojo to resign along with his entire war cabinet. The Emperor's supreme naval advisor, Admiral Nagano, said that 'Hell is on us'. The fall of the Marianas meant that the Americans were now in a position from where they could cut the Japanese Empire in half - forces ranging west could hit the Philippines and threaten the sea routes to the southern resource area, while B-29 bombers based on the islands could reach the Japanese Home Islands.