Saipan triggers the fleet battle

The battle becomes unavoidable when American strikes and the Saipan landing make the Marianas threat unmistakable to Japan. Capturing Saipan would place American strategic bombers within range of the Japanese home islands, forcing Tokyo to commit the mobile fleet it had preserved for decisive battle. U.S. submarines then reported Japanese movements, while a broken radio-silence message allowed direction finding to provide an approximate position. Spruance moved the fast carriers west of Saipan but left older battleships, escort carriers, cruisers, and escorts with the invasion force. His task was therefore to block the Japanese route to the amphibious objective, not chase a carrier duel far to the west.