The powerful forces of the United States Navy (USN), Marine Corps, and Army advanced inexorably against Imperial Japan in 1944. Following massive interdiction of Japanese merchant shipping by American submarines and multiple naval victories, the Americans stood poised to liberate the Philippines, then move on to locations closer to the Japanese home islands.
In early 1944, arguments raged over the best approach to the "strategic triangle" created by Formosa, Luzon, and China. Finally, on March 12th, the Joint Chiefs of Staff - consisting of Admirals William D. Leahy and Ernest J. King, and Generals George C. Marshall and Henry H. "Hap" Arnold - issued a directive picking the next target: "[T]he most feasible approach to the Formosa-Luzon-China area is by way of Marianas-Carolines-Palau-Mindanao area, and that the control of the Marianas-Carolines-Palau area is essential to the projection of our forces into the former area, and their subsequent effective employment therefrom." The Americans' plans focused on three islands near the southern end of a 15-island, north to south aligned island chain: Saipan, Tinian, and Guam. These islands, relatively large, offered space for the construction of large air bases within strategic bomber range of Japan itself, as well as closer targets.
The Japanese also recognized the strategic importance of the Mariana Islands, and Saipan in particular, given its location just 1,272 miles from Tokyo itself. This would place the Japanese capital well within the 3,250 mile range of the Boeing B-29 Superfortress. With these facts in their possession and the Marianas as one of the Americans' most logical next choices, the Japanese worked to move both reinforcements and materials for new fortifications to the southern Marianas in early 1944.
Nevertheless, deadly USN submarines with determined crews seriously hampered these efforts. On February 29th, the USS Trout (SS-202), a Tambor-class submarine skippered by Lieutenant Commander Albert Clark, sank the transport Sakito Maru on its way to Saipan. This killed 2,420 men on board the ship, including a considerable portion of the IJA 18th Infantry Regiment. The Trout itself perished with all hands, either to depth charges from the destroyer Asashimo or from one of its own Mark XVIII torpedoes running in a circle and hitting it.
Many other transports full of vital war materials went to the bottom as the USN submarines interdicted Japanese ship traffic to the Marianas. Major General Ikeda Keiji complained, "We cannot strengthen the fortifications [...] unless we can get materials suitable for permanent construction. Specifically, unless the units are supplied with cement, steel reinforcements for cement, barbed wire, lumber, etc., [...] no matter how many solders there they can do nothing [...] but sit around with their arms folded, and the situation is unbearable." (Denfeld, 1997, 17).
The USN thus used its submarine superiority, to which the Japanese had no effective counter, to greatly hamper efforts to fortify and reinforce the Marianas. With that, the stage was set for the kind of deadly amphibious operations that would take place not only on the Mariana and Palau Islands but also Iwo Jima and Okinawa after it. As a result, the campaign helped persuade President Truman to use the atomic bombs against Japan, and the planes that delivered them to Hiroshima and Nagasaki would end up taking off from airfields constructed by the victorious Americans in the wake of their success in the Marianas.