Operation Forager, the American invasion of the Mariana Islands in June and July 1944 during World War II, was the largest amphibious operation of the Pacific War to that date. It involved the movement and support of a fleet and 166,000 ground troops over extremely long distances. Intervention and disruption by Japanese air and naval forces was anticipated. This meant that not only did the forces ashore have to be supplied with enough resources to continue operations if unloading was interrupted, but a fleet of warships had to be maintained at sea for up to four months.
A crucial problem was the provision of sufficient shipping. A shortage of ships and cargo space forced units to leave some vehicles and equipment behind, and some units had to travel on separate ships from their equipment. Unloading was facilitated by the use of wooden pallets, but not all cargo ships had the space or equipment to accommodate them. On Guam the reef prevented landing craft from directly accessing the beach and pallets had to be broken up and stores transferred to DUKWs or Landing Vehicles, Tracked, (LVTs) on the reef.
Two dozen replenishment oilers were on hand to enable the fleet to refuel at sea. For the first time in the Central Pacific Area, the Japanese targeted oilers. With the fleet at sea for over four months, many stores aboard ran low. These were replenished by general stores issue ships carrying 5,000 different items. The Battle of the Philippine Sea and staunch Japanese resistance ashore on Saipan and later Guam resulted in a longer campaign than anticipated, with increased calls for naval gunfire support, and consequently prodigious ammunition consumption. Most resupply ships were not vertically loaded, which made retrieving the most urgently needed items more problematic. Ammunition ships were called forward to replenish the fleet off Saipan while the battle still raged on shore, a risky action as Japanese air attacks occurred almost every day.
After the battle, the Mariana Islands were developed into a major naval and air base. Hospitals on the islands received wounded from the Battle of Iwo Jima and the Battle of Okinawa. Boeing B-29 Superfortress bombers were based on the islands and conducted air raids on Japan, including the bombing of Tokyo on 10 March 1945 and the atomic bombings of Hiroshima and Nagasaki.
Background Operation Forager, the U.S. invasion of the Mariana Islands during World War II, involved the movement and support of a fleet of ships and two corps of ground troops over a distance of from Hawaii. It was the largest amphibious operation of the Pacific War to that date. The American forces were part of Admiral Raymond A. Spruance's Fifth Fleet. The Joint Amphibious Forces (Task Force 51) were led by Vice Admiral Richmond K. Turner and the expeditionary troops (Task Force 56) by Lieutenant General Holland M. Smith.
The Mariana Islands were mountainous, with a total land area of about . Unlike the small, low-lying coral atolls captured in the Gilbert and Marshall Islands campaign, there was little prospect of the islands being overrun quickly and prolonged fighting was a possibility. Accordingly, the assault troops carried 32 days' supply of rations, 30 days worth of medical supplies, 20 days' supply of fuel, and seven units of fire for all weapons, except anti-aircraft guns, for which ten units of fire were taken.
This represented per soldier, considerably more than the per soldier of the US forces engaged in the invasion of Sicily the year before. The supply lines to Sicily were reasonably secure, whereas for Forager, intervention and disruption by Japanese air and naval forces was anticipated. Not only did the forces ashore have to be supplied, but a fleet of warships had to be maintained at sea for up to four months. Plans for the assault also had to be coordinated with those for base development, as this was the overall objective of the campaign.
The first objective in Operation Forager was the occupation of Saipan, which commenced on 15 June 1944, after a two-day naval bombardment, around the same time the Allies were invading France in Operation Overlord. The 2nd Marine Division, 4th Marine Division, and the Army's 27th Infantry Division engaged in costly fighting before the island was declared secure on 9 July. In addition to the fighting on land, there was a naval battle, the Battle of the Philippine Sea, on 19-20 June. On 24 July 1944, the 4th Marine Division landed on nearby Tinian, supported by aerial and naval bombardment and the guns of the XXIV Corps Artillery, firing across the strait from Saipan. Instead of landing in the southwest, the Marines landed on the northwest coast, where two very small beaches were lightly defended. The island was declared secure on 1 August, but mopping-up operations continued until 1 January 1945.
The assault on Guam was originally scheduled for 18 June, just three days after the landing on Saipan. The Battle of the Philippine Sea and stubborn resistance by the unexpectedly large Japanese garrison on Saipan led to the invasion being postponed for over a month to allow the Army's 77th Infantry Division to be brought in from Hawaii. American forces landed on Guam on 21 July, and on 10 August organized Japanese resistance was declared ended, though as many as 10,000 Japanese soldiers were still at large.
Naval logistics By using up the most perishable stores first, a ship's supply officer could maintain a warship at sea for up to two months, but Operation Forager required the fast carrier task groups to be at sea for more than four. The ship's stores therefore had to be replenished by general stores issue ships (AKSs). The ships of the fleet carried about 40,000 different items, but the AKSs carried only the 5,000 most urgent stores.
Shipping The provision of shipping to maintain the force was a critical problem. A staff study estimated this would require 120 cargo ships and 58 troop transports, of which only 18 and 22 respectively were on hand in April 1944. For the invasion of Saipan, 110 transport vessels were available. They carried of cargo. Staging areas were widely separated; the 2nd Marine Division loaded at Hilo on the island of Hawaii; the 4th Marine Division at Kahului on Maui, and the 27th Infantry Division on Oahu. All the combatant ships involved in the operation were American, as were all but three auxiliary ships: there were two Dutch merchant vessels and a Norwegian whaler factory ship converted for use as an oil tanker. The Northern attack force for Saipan and Tinian consisted of 71,034 soldiers, sailors and marines; the Southern Attack Force for Guam, of 56,537. In addiation, there were 9,250 troops for the Guam garison, 23,616 for Saipan and 5,235 for Tinian. Thus, 165,672 troops were embarked.
Combat loading, in which the most urgently required cargo was stowed in ships' holds so as to be immediately accessible, was favored for ships carrying assault troops, but was not always possible due to the shortage of shipping. Some units of the V Amphibious Corps and the XXIV Corps Artillery found themselves traveling separately from their unit equipment, a highly undesirable scenario in a combat zone, but fully combat loading the entire force would have meant leaving up to 35 percent of their equipment behind. Even then, there was insufficient cargo space, and the V Amphibious Corps was forced to make cuts to unit equipment. This fell most heavily on transport, which was bulky. Only 94 percent of ambulances, 83 percent of trucks, 71 percent of trailers and 75 percent of tractors could be taken to Saipan.
This was partially offset by the allocation of 185 DUKWs. Each division had a DUKW company: the 1st Amphibian Truck Company for the 2nd Marine Division, the 2nd Amphibian Truck Company for the 4th Marine Division, and a provisional amphibian truck company for the 27th Infantry Division. In addition, the 477th Amphibian Truck Company was attached to the XXIV Corps Artillery. For Guam, the 3rd Marine Division and the 1st Provisional Marine Brigade shared the III Amphibious Corps Motor Transport Battalion, with its Company C assigned to the brigade. When a typhoon struck on 29 July, an LST broached and Landing Vehicles, Tracked, (LVTs) and landing craft were unable to operate, but the DUKWs proved more seaworthy, and they were able to continue delivering supplies.
The V Amphibious Corps Headquarters ordered that 25 to 50 percent of all supplies and two to five units of fire of ammunition be loaded on pallets. This order was embraced by the 27th Infantry Division, which put between 80 and 90 percent of its supplies on pallets, but the 2nd and 4th Marine Divisions were less enthusiastic about the idea. The former put no more than 25 percent of its supplies on pallets, and the latter only palletized 10 to 15 percent. The 3rd Marine Division and the 1st Provisional Marine Brigade did not put any of their supplies on pallets, but the 77th Infantry Division complied with the directive and constructed more than 5,000 wooden pallets, although about 1,000 pallet loads were broken up because they did not fit into the available cargo space or because a ship lacked the equipment to handle pallets.
On Smith's recommendation, resupply ships sailing from the United States were "block loaded", with a mixture of common stores. This proved not only to be inefficient in the use of cargo space, but presented difficulty in unloading, as each type of supply had to be completely unloaded before the next could be accessed. In his evaluation, Turner recommended the Navy revert to commodity loading, where each ship carried a particular type, which facilitated selective unloading of stores.
Food and water The Southern Attack Force was mounted from the South Pacific Area, so most of its initial complement of fresh provisions came from New Zealand, while those for the Northern Attack Force, destined for the assaults on Saipan and Tinian, came from Hawaii and the West Coast of the United States. Once the operation was under way, resupply was from the West Coast through the bases in Hawaii, and on Majuro, Kwajalein and Eniwetok. Rations for Army units were drawn from Army stocks while those for the Navy and Marine Corps came from Navy stocks. Resupply directly from the United States was necessary because the base facilities in Hawaii were insufficient to handle the volume of supplies required for mounting simultaneous operations against Saipan and Guam. Ships returning from the forward areas to supply points transferred excess stores not required for the return voyage to other vessels.
The supply of fresh produce depended on the availability of reefer ships, cargo ships with refrigerated holds. These were specially designed to carry a mixture of fresh and dry provisions, and to permit selective discharge of a balanced cargo so one ship after another could be revictualed without having to unload the reefer's cargo and re-stow it. The Service Force had nine reefers, as the majority of the Navy's reefers were serving with the Atlantic Fleet. The Pacific Fleet's reefers were therefore supplemented by ships of the War Shipping Administration. These were large vessels with Merchant Marine crews that could carry up to of refrigerated produce, enough for 30 days' supply for 90,000 soldiers; but their holds and hatches were not organised for selective discharge, so they had to unload into warehouses, and Saipan had no such facilities until 15 August. Nonetheless, during Operation Forager, the Service Force managed to supply fresh produce to all combatant ships on five days out of six, and to the forces ashore for one day in three.