Meanwhile, Japan—determined to defend Saipan, no matter how high the cost—was sending Admiral Jisaburō Ozawa's powerful First Mobile Fleet from the Sulu Islands to the Marianas to sink the warships of Admiral Raymond A. Spruance's 5th Fleet and to annihilate the American troops who had fought their way ashore on Saipan. Soon after the Japanese task force sortied from Tawi Tawi on the morning of 13 June, American submarine spotted and reported it. Other submarines—which from time to time made contact with Ozawa's warships—kept Spruance posted on their progress as they wended their way through the islands of the Philippines, transited San Bernardino Strait, and took part in the Battle of the Philippine Sea.