After the conquest of the Gilbert Islands in November 1943 the obvious next move for Nimitz was to strike at the Marshall Islands and thus take one more step toward Japan. The Marshalls consist of many widely scattered atolls or island groups. They had become a Japanese mandate after World War I and had been secretly fortified long before the attack on Pearl Harbor. By the end of 1943, however, the Japanese had concluded that the Marshalls could not be successfully defended against a determined attacker, though resistance could delay the Allied advance and prove costly to the Allies in both men and matériel. This latter point was of considerable importance because the Allies were operating thousands of miles from their sources of supply. The Joint Chiefs of Staff directed Nimitz to move in January 1944 against three of the atolls that were known to be fortified. The admiral decided to move against only one, Kwajalein, which, being 60 miles (almost 100 km) long and 20 miles (32 km) wide, is one of the world’s biggest atolls.

In planning the attack on Kwajalein the lessons of Tarawa were carefully studied. This time there would be far more naval gunfire to crush concrete emplacements. The powerful armada sent against Kwajalein included four carrier task groups under the command of Adm. Marc A. Mitscher. The Allies were able to maintain complete air superiority in the contested area and to bomb the defenders in their fortified positions. No amount of preliminary bombardment could dislodge the defenders, however; that task could be accomplished only by infantry armed with grenades, demolition charges, and flame throwers. Within a week after the landing of the U.S. 7th Infantry Division and the 4th Marine Division on Kwajalein on January 31, 1944, most of the resistance had been overcome, and with far fewer U.S. casualties than at Tarawa. The Japanese resistance on Kwajalein had been overcome so readily, in fact, that Mitscher was able to move his forces on to Eniwetok on February 17.

To support these landings in the Marshalls, the U.S. Pacific Fleet launched a series of day-and-night air attacks on the Japanese base at Truk (now Chuuk) on February 17–18, 1944. Sometimes called “the Gibraltar of the Pacific,” Truk was the capital of the Caroline Islands that lay due west of Kwajalein. It had an excellent harbour and had served as the headquarters of the Japanese Combined Fleet. Carried out by Mitscher’s Task Force 58, the raids on Truk were among the most successful of the war, destroying 275 Japanese aircraft and sinking 200,000 tons of merchant shipping. Most of the Japanese combat ships, warned of impending attack by a reconnaissance plane, had left the anchorage and fled to a safer haven in the Palau Islands. On orders from the Joint Chiefs of Staff, Truk was given the same treatment as Rabaul: being too weak and isolated to be a serious threat it was bypassed.

The Pacific Fleet, having taken Kwajalein and Eniwetok and neutralized Truk, was face to face with its next objective, the Mariana Islands. The numerous islands in this group stretch in a 400-mile (approximately 640-km) arc from Guam northward toward Iwo Jima. They stand roughly halfway between the Gilberts and the Japanese home islands, approximately 1,500 miles (2,400 km) from Tokyo. The four largest islands in the group, and the only ones of military importance in 1944, were Guam, Saipan, Tinian, and Rota. Guam, which the U.S. had gained in 1898, had fallen to the Japanese shortly after Pearl Harbor. The plan to capture these four islands was one of the most ambitious in the history of amphibious warfare. It was to be carried out at vast distances from permanent bases: 3,500 miles (more than 5,600 km) from Pearl Harbor and 1,000 miles (1,600 km) from Eniwetok. The operation required the services of more than 500 ships of all types, carrying approximately 125,000 troops. Involving a long sea voyage to attack a small island, it stood in striking contrast to the Normandy Invasion that took place in Europe in the same month (June 1944). Both required the marshaling of vast air, sea, and ground resources but the Normandy Invasion entailed the crossing of only the narrow English Channel; its staging area was an island and its objective was the invasion of the European continent.

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In preparation for the assault on Saipan, Task Force 58 launched intensive air attacks on the island, and battleships bombarded the defenses with their big guns. The troops were carried ashore on amphibious tractors or small boats while offshore naval vessels provided covering fire and aircraft attacked the area behind the beaches. On June 15, 1944, the 2nd and 4th Marine divisions went ashore against stubborn opposition from approximately 30,000 Japanese defenders. The defenders were well protected in caves and bunkers and had not been badly hurt by the preliminary bombardment. The result was that the Marines suffered heavy casualties and after five days the Army’s 27th Division was sent ashore to reinforce them. Gradually compressed into smaller and smaller pockets, the Japanese themselves ended most organized resistance with a suicidal counterattack on July 7, the largest of its kind during the war.

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After the U.S. forces had thrown back this attack, the two Japanese commanders on the island committed suicide. To the astonishment of U.S. troops, hundreds of Japanese civilians also took their own lives, some by jumping off the northern cliffs onto the reefs below. When news of the loss of Saipan reached Tokyo, Prime Minister Tōjō Hideki and his entire cabinet resigned. Many in the Japanese High Command recognized that the Marianas were effectively the last line of defense before the Allies reached the home islands, but none dared voice that opinion. Tojo was succeeded by Koiso Kuniaki, who pledged to continue the increasingly doomed effort.

Air power advocates viewed the victory on Saipan as perhaps the most significant event in the Pacific War. It enabled the U.S. to establish air bases for B-29 heavy bombers, which had been developed for the specific purpose of bombing Japan. The first flight of 100 B-29s took off from Saipan on November 24, 1944, and bombed Tokyo. This marked the first bombing raid on the Japanese capital since the Doolittle Raid of April 18, 1942.

Perhaps of greater importance than the landing on Saipan was the naval battle that accompanied it. The Japanese Combined Fleet, after steaming northward from anchorages in the Philippines and the East Indies, converged on the Marianas. Since its heavy losses in aircraft and pilots at Rabaul, the Japanese Navy had avoided combat while it formed and trained new air units. Now it felt that the time had come to destroy the U.S. Pacific Fleet in one great battle. Command of the Japanese Fleet was given to Adm. Ozawa Jisaburō. Warned by submarines of the approach of the Japanese armada, Adm. Raymond Spruance, commander of the U.S. 5th Fleet, postponed the invasion of Guam and prepared to meet the Japanese ships. Ozawa, with only 9 carriers against 15 for the U.S., was obviously outclassed in naval air power but he believed that land-based aircraft from Guam, Rota, and Yap would bring the two forces to near parity.

The Battle of the Philippine Sea took place west of the Marianas and has been called the greatest carrier battle of the war. On the morning of June 19 Ozawa sent 430 planes in four waves against Spruance’s ships. The result for the Japanese was a disaster. U.S. airmen shot down so many Japanese planes that the engagement was subsequently dubbed “the Great Marianas Turkey Shoot.” In the first day of the battle the Japanese lost more than 300 planes and two carriers. As the fleet retired northward toward safe harbour at Okinawa it lost another carrier and nearly 100 more planes. During the two days of battle, U.S. losses totaled 130 aircraft and some damage to ships. The poor showing by the Japanese has been attributed to many factors, but two may be singled out for special mention: men and matériel. Some Japanese pilots went into action with as little as three months of training while many U.S. pilots had spent two full years in training. Japanese planes were highly maneuverable and had a longer range than U.S. planes but they were inferior in several respects, particularly in their inadequate armour protection and lack of self-sealing fuel tanks. U.S. submarines also played an important but less publicized role in providing U.S. commanders with intelligence of enemy movements and in sinking Japanese ships.

After Saipan, Nimitz’s forces in the Central Pacific proceeded with the occupation of the other major islands in the Marianas. Guam was invaded on July 21 and was finally secured after heavy fighting. Tinian was attacked on July 24 and occupied after light opposition. Tinian would later earn a place in history as the base for the plane that dropped the first atomic bomb on Japan. The entire Marianas campaign cost U.S. forces about 4,750 men killed while the Japanese lost approximately 46,000 men killed or captured.

After the occupation of Guam, U.S. forces had two routes to choose from for their next move: one to the north to Iwo Jima, aiming at Japan, and the other to the west where the Palau Islands stood within striking distance of the Philippines. The attack on Iwo Jima was postponed until February 1945 so that Nimitz’s forces could invade the Palaus at the same time that MacArthur was seizing Morotai. On September 15, 1944, the U.S. 1st Marine Division went ashore on Peleliu in the Palaus and two days later the 81st Infantry Division landed on Angaur to the south. Fighting was fierce and casualties ran high on both sides, but the outcome was never really in doubt. When organized opposition in the Palaus ceased in November it could be said that for the Allies the road to the Philippines was open from both the south and the east.