The Japanese had been in possession of the Marianas, with the exception of Guam, since 1914 when the island group was seized from the Germans. During the 1930's considerable resources had been employed by the Japanese to build up Saipan for both naval and air capability, with increasing secrecy from the international community. Saipan was developed into a significant strategic location for the Japanese. During the latter part of the war the US was establishing a strategic direction to take in the Pacific, eventually deciding on the Central Pacific Drive as the final course of action, despite the scepticism of some who desired to bypass the islands. The Marianas was chosen as potential airbase to station B-29 bombers to attack the Inner Zone of the Japanese Empire. Previous plans had sought to staton B-29's in China, but this was met with skepticism.

The Americans had intelligence that the Japanese were anticipating a US attack on the Palau Islands and not Saipan and focussed their defensive efforts there. A shortage of building materials to the island had also left the defences within Saipan undeveloped.

On the 4th of April 1944 Admiral Turner personally put together the plan and objectives for the Marianas, codenamed 'Forager'. He chose the island of Saipan to invade first, as it had ideal air fields and was 100 miles closer to Japan, key elements in meeting the strategic requirement of keeping the pressure on Japan with heavy bombers .

Turners mission statement read: "The objective is the capture of Saipan, Tinian and Guam, in order to secure control of the sea lines of communications throughout the Central Pacific for preparation and support of future attacks on the Japanese homeland."

On the 20th of March 1944 the CINCPOA (Commander in Chief, Pacific Ocean Area) FORAGER, Joint Staff Study was issued stating that the purpose of the operation was to:"attack the enemy's sea-air communications, support operations for the neutralization of by-passed Truk, initiate B-29 bombing of the Japanese home islands, and support further offensives against the Palaus, Philippines, Formosa and China". Broadly the intent was to "maintain unremitting military pressure against Japan" and to "extend our control over the Western Pacific." Saipan was chosen to be taken first as it could facilitate this strategy quickly, with the other islands to follow.

Sources:

Saipan: The Beginning of The End. By Major Carl W. Hoffman USMC.

Saipan and Joint Operations. By Lieutenant Colonel Edward T. Buckley Jr. US Army War College.

History Of U.S. Marine Corps Operations In WWII: Volume III, Central Pacific Drive. Historical Branch, USMC.