However, the amphibious forces first headed for Saipan -— regarded by some as the key to Japan's inner defenses. The assault on that island on 15 June resulted in the Japanese Fleet's challenging the Americans for the first time since the Battle of Midway, two years before. In the ensuing Battle of the Philippine Sea, on 19 and 20 June 1944, the enemy suffered heavily, losing three carriers. In the two-day battle, the Japanese Navy lost nearly 92 percent of its carrier-based aircraft (395 planes) as opposed to 130 planes lost by TF 58, the fast carrier task force under Vice Admiral Marc A. Mitscher.