Battle of The Philippine Sea - Background

Background

On 19 April 1943, Admiral Mineichi Koga succeeded Isoroku Yamamoto as Commander in Chief of the Combined Fleet. Under his direction Imperial Japanese Navy Headquarters had hoped to return to the offensive in early 1944; however, this soon proved impracticable.

The Japanese war plan had from the start always been to inflict such severe and painful losses upon the Americans that the American public would become war weary and demand a negotiated settlement. American technological advances and material production capacity made this objective increasingly difficult to achieve. By the end of 1942 the Allied navies had overcome most of the technological edges Japan's ships and planes had held at the start of the war. Furthermore, by mid-1943 mass production of updated ships and improved aircraft began to tip the balance of forces in the favor of the Allies. Allied educational training practices similarly adapted to new developments, along the way totally revising the fleet operations with the parallel development of both the Combat Information Center and doctrine, training, and practices to get the most out of the new communications and sensor technologies.

Japan's ability to prosecute the war was increasingly hampered by a severe shortage of qualified pilots. Japanese pre-war pilot training was oriented toward turning out relatively small numbers of highly-trained pilots, and the training organizations were not well prepared to turn out the large numbers of pilots needed to replace wartime losses. Most of Japan's more skillful and experienced pilots had died during two years of near continual engagements as the U.S. stabilized the theater. The IJN's small cadre of highly-trained carrier pilots had not recovered from losses sustained at the 1942 battles at Coral Sea and Midway. The war was draining down the talent level of Japan's land based air forces as well, with progressive air fleet activities in the New Guinea campaign and the Guadalcanal campaign drastically reducing the average skill level of Japan's land and sea based pilots by taking out the best of them in grinding battles. In 1943-44 other island war engagements exacerbated the problem from the Japanese point of view, as the U.S. fast carrier task groups began purposefully to raid and pound Japanese island strongholds specifically to reduce their air power and ability to protect shipping.

The Japanese commanders saw Guam, Tinian, and Saipan as part of an inner air-sea defense ring of land-based fighter and bomber bases surrounding and protecting their home islands. The American “island hopping campaign” was proceeding faster than anticipated, disrupting Japanese Sea lines of communications (SLOCs) and destroying or isolating one island outpost after another. By late 1943 Japan's 'outer' defensive ring had been thoroughly punctured, with the U.S. fleet working its way over or past such strongholds in their steady progression across the Central Pacific islands. The Imperial Staff calculated the Marianas would come under similar attack, which would put the Japanese home islands within effective range of American bombers. While revising their offensive ambitions the Japanese staff also ordered prepared defenses and planning for various island bases they believed would be likely targets.

IJN planning evolved into a carrier-assisted battle fleet attack on the U.S. invasion fleet as it was pinned down protecting transports and newly-landed forces. The battle fleet sortie concept was very similar to that intended by the late Admiral Yamamoto in the battle of Midway. Many of the officers who had planned Midway were still on the General Staff of the Combined Fleet. In their planning it was believed that the lack of offensive air power could be addressed by deploying strong land-based air forces—a natural connection, for Japan had taken pains to build many bases that could control the SLOCs around them. The initial plan was to attack the U.S. Pacific Fleet in early 1944, whenever it launched its next offensive.

In March 1944, Japan's combined fleet again lost its commander in an aircraft, this time during a Pacific hurricane. Under direction of the newly appointed Commander in Chief of the Combined Fleet, Admiral Soemu Toyoda, a finalized plan for the new conditional 'offensive'—dubbed "Plan A-Go", or "Operation A-Go"—was adopted in early June 1944, then within weeks, quickly adapted to trap the American fleet now detected heading for Saipan.