The Battle of Saipan was an amphibious assault launched by the United States against the Empire of Japan during the Pacific campaign of World War II between 15 June and 9 July 1944. The initial invasion triggered the Battle of the Philippine Sea, which effectively destroyed Japanese carrier-based airpower, and the battle resulted in the American capture of the island. Its occupation put the major cities of the Japanese home islands within the range of B-29 bombers, making them vulnerable to strategic bombing by the United States Army Air Forces. It also precipitated the resignation of Hideki Tōjō, the prime minister of Japan.
Saipan was the first objective in Operation Forager, the campaign to occupy the Mariana Islands that got underway at the same time the Allies were invading France in Operation Overlord. After a two-day naval bombardment, the U.S. 2nd Marine Division, 4th Marine Division, and the Army's 27th Infantry Division, commanded by Lieutenant General Holland Smith, landed on the island and defeated the 43rd Infantry Division of the Imperial Japanese Army, commanded by Lieutenant General Yoshitsugu Saitō. Organized resistance ended when at least 3,000 Japanese soldiers died in a mass gyokusai attack, and afterward about 1,000 civilians committed suicide.
The capture of Saipan pierced the Japanese inner defense perimeter, and forced the Japanese government to inform its citizens for the first time that the war was not going well. The battle claimed more than 46,000 military casualties and at least 8,000 civilian deaths. The high percentage of casualties suffered during the battle influenced American planning for future assaults, including the projected invasion of Japan.
Background
American strategic objectives
Up to early 1944, Allied operations against the Japanese military in the Pacific were focused on securing the lines of communication between Australia and the United States. These operations had recaptured the Solomon Islands, eastern New Guinea, western New Britain, the Admiralty Islands, and the Gilbert and Marshall Islands.
To defeat Japan, Admiral Ernest J. King, Commander in Chief, United States Fleet, sought to execute War Plan Orange, which the Naval War College had been developing for four decades in the event of a war. The plan envisioned an offensive through the Central Pacific that originated from Hawaii, island-hopped through Micronesia and the Philippines, forced a decisive battle with the Japanese Navy, and brought about an economic collapse of Japan.
As early as the Casablanca Conference in January 1943, King presented the case to the Combined Chiefs of Staff for an amphibious offensive in the Central Pacific – including the Marshall Islands and Truk – that would capture the Mariana Islands. He stated that the occupation of the Marianas – specifically Saipan, Tinian, and Guam – would cut the sea and air route from the Japanese home islands to the western Pacific, but the Combined Chiefs of Staff made no commitment at the time. General Douglas MacArthur, Supreme Commander of Allied Forces in the Southwest Pacific Area, objected to King's proposed Central Pacific offensive. He argued that it would be costly and time-consuming and would pull resources away from his drive in the Southwest Pacific toward the Philippines.
At the Quebec Conference in August 1943, King continued to advocate for including the Marianas in a Central Pacific offensive. He suggested that the strategic importance of the Marianas could draw the main Japanese fleet out for a major naval battle. King's advocacy gained support from General Henry H. Arnold, Chief of the Army Air Forces, who wanted to use the newly developed B-29 bomber. The Marianas could provide secure airfields to sustain a strategic bombing offensive as the islands put much of Japan's population centers and industrial areas within the B-29's 1,600 mi (2,600 km) mile combat radius. At the Cairo Conference in November 1943, the Combined Chiefs of Staff supported both MacArthur's offensive in the Southwest Pacific and King's in the Central Pacific, adding the Marianas as an objective for the Central Pacific offensive and setting 1 October 1944 as the date for their invasion.
Admiral Chester Nimitz, Commander-in-Chief, Pacific Ocean Areas, led the Central Pacific offensive. In January–February 1944, the Marshall Islands were quickly captured and a massive American carrier-based air attack on Truk demonstrated that it could be neutralized and bypassed. On 12 March 1944, the Joint Chiefs of Staff moved the date of the invasion up to 15 June with the goal of creating airfields for B-29s and developing secondary naval bases. Nimitz updated the plans for the Central Pacific offense, codenamed Granite II, and set the invasion of the Marianas, codenamed Forager, as its initial objective. Saipan would be the first assault.
Japanese strategic plan
The Japanese Imperial War Council established the "Absolute National Defense Zone", Zettai Kokubōken) in September 1943, which was bounded by southern half of Sakhalin and the Kuril Islands, the Bonin Islands, the Marianas, Western New Guinea, Malaya, and Burma. This line was to be held at all costs if Japan were to win the war. The Marianas were considered particularly important to protect, as their capture would put Japan within bombing range of the B-29 bomber and allow the Americans to interdict the supply routes between the Japanese home islands and the western Pacific.
The Imperial Japanese Navy planned to hold the defense line by defeating the United States fleet in a single decisive battle, after which, the Americans were expected to negotiate for peace. The Japanese Submarine Fleet (6th Fleet), commanded by Vice Admiral Takeo Takagi, whose headquarters was on Saipan, would screen the line. Any American attempt to breach this line was to serve as the trigger to start the battle. The defense forces in the attacked area would attempt to hold their positions, while the Japanese Combined Fleet struck the Americans, sinking their carriers with land-based aircraft and finishing the fleet off with surface ships. As part of this plan, the Japanese could deploy over 500 land-based planes – 147 of them immediately in the Marianas – that made up the 1st Air Fleet under the command of Vice Admiral Kakuji Kakuta, whose headquarters were on Tinian.
History and geography
Saipan and the other Mariana Islands were claimed as Spanish possessions by conquistador Miguel López de Legazpi in 1565. After Spain's defeat in the Spanish-American War, Saipan was sold to Germany in 1899. The island was occupied by the Japanese in 1914 during World War I, who made it the administrative center for the Mariana Islands – Saipan, Tinian, and Rota – that were part of Japan's South Seas Mandate.
Saipan has a tropical marine climate with a mean annual temperature of 85 °F (29 °C) in the lowlands and 78 °F (26 °C) in the highlands. Though the island has a mean rainfall between 81 inches (210 cm) and 91 inches (230 cm) per year, the rainy season does not begin until July.
Unlike the small, flat coral atolls of the Gilberts and Marshalls,
Saipan is a volcanic island with diverse terrain well suited for defense. It is approximately 47 sq mi (122 km2),
and has a volcanic core surrounded by limestone. In the center of the island is Mount Tapotchau, which rises to 1,554 ft (474 m). From the mountain, a high ridge ran northward about 7 miles (11 km) to Mount Marpi. This area was filled with caves and ravines concealed by forest and brush, and the mountainous terrain would force tanks to stay on the island's few roads, which were poorly constructed.
The principal airfield of the Marianas, Aslito Field, was located on the southern half of the island. It served as a repair stop and transit hub for Japanese aircraft headed toward other parts of the Pacific.
This half of the island was flatter, but covered with sugarcane fields because the island's economy became focused on sugar production after the Japanese government had taken over Saipan from Germany in 1914. About 70% of Saipan's acreage was dedicated to sugarcane. It was so plentiful that a narrow-gauge railway was built around the perimeter of the island to facilitate its transportation. These cane fields were an obstacle to attackers; they were difficult to maneuver in and provided concealment for the defenders.
Saipan was the first island during the Pacific War where the United States forces encountered a substantial Japanese civilian population, and the first where U. S. Marines fought around urban areas. About 26,000 to 28,000 civilians lived on the island, primarily serving the sugar industry. Most of them were Japanese subjects, primarily from Okinawa and Korea; a minority was Chamorro people. The largest towns on the island were the administrative center of Garapan, which had a population of 10,000, Charan Kanoa, and Tanapag. They were on the western coast of the island, which was where the best landing beaches for an invasion were located.
Opposing forces
American invasion force
Nimitz, the commander of the Pacific Fleet, assigned Admiral Raymond Spruance, commander of the Fifth Fleet, to oversee the operation. Vice Admiral Richmond K. Turner, Commander, Joint Amphibious Forces (Task Force 51) oversaw the overall organization of the amphibious landings of the Mariana Islands; he also oversaw the tactical command of the landing on Saipan as Commander, Northern Attack Force (TF 52). Once the amphibious landings were completed, Lieutenant General Holland M. Smith, Commanding General, Expeditionary Troops (Task Force 56), would oversee the ground forces for all of Forager; he would also oversee the ground combat on Saipan as Commander, Northern Troops and Landing Force.
The Northern Troops and Landing Force was built around the V Amphibious Corps, which consisted of the 2nd Marine Division commanded by Major General Thomas E. Watson and 4th Marine Division commanded by Major General Harry Schmidt. The 27th Infantry Division commanded by Major General Ralph C. Smith was held as the Expeditionary Troops reserve for use anywhere in the Marianas. Over 60,000 troops were assigned to the assault. Approximately 22,000 were in each Marine division and 16,500 were in the 27th Infantry Division.
The invasion fleet, consisting of over 500 ships and 300,000 men, got underway days before the Allied forces in Europe invaded France in Operation Overlord on 6 June 1944. It was launched from Hawaii, briefly stopping at Eniwetok and Kwajalein before heading for Saipan. The Marine divisions left Pearl Harbor on 19–31 May and met at Eniwetok on 7–8 June; the 27th Infantry Division left Pearl Harbor on 25 May and arrived at Kwajalein on 9 June. The 15 aircraft carriers of the Fast Carrier Task Force (Task Force 58) commanded by Vice Admiral Marc A. Mitscher, which would provide support for the invasion, left Majuro for Saipan on 6 June.
The invasion force included 150 tanks, over 100 of which were M4 Shermans. The M4 Sherman tank was superior to the medium tank used by the Japanese, the Type 97. Shermans were primarily used to support infantry and was considered one of the most effective weapons for destroying enemy emplacements. Flamethrowers were extensively used. Smith had seen the need for motorized flamethrowers and had requested that the Army's Chemical Warfare Service (CWS) in Hawaii install them in M3 Stuart tanks. Seabees with the CWS had 24 tanks, nicknamed "Satans", converted to flamethrowing in time for the invasion. They were very effective for destroying pillboxes, cave defenses, buildings, cane fields, and brush.
Japanese defense preparations
American intelligence had estimated that there would be between 15,000 and 18,000 Japanese troops on Saipan at the time of the invasion. In actuality, there were double that number. Nearly 32,000 Japanese military personnel were on the island, including 6,000 naval troops. The two major army units defending the island were the 43rd Division commanded by Lieutenant General Yoshitsugu Saitō and the 47th Independent Mixed Brigade commanded by Colonel Yoshira Oka. Both units were assigned to the Thirty-First Army under the overall command of Lieutenant General Hideyoshi Obata.
The Japanese hurriedly reinforced the island before the invasion, but many of the troop transports were sunk by U.S. submarines. For example, five of the seven ships transporting the 43rd Division were sunk. Most of the troops were saved, but the majority of their equipment–including hats and shoes–was lost, reducing their effectiveness. Many soldiers were the stranded survivors of sunken ships headed to other islands. About 80 tanks were on the island, substantially more than the Americans had encountered in previous battles with the Japanese.