The Mariana and Palau Island campaigns, otherwise Operation Forager, was an Allied advance in the Pacific War. The USA landed Marines to capture airfields in the Marianas, and secure their flank in Palau. The operation began the campaign at Saipan, and it was there that the opening battles emerged.
As the U.S. invasion fleet approached Saipan, the Imperial Japanese Navy sent a fleet of aircraft carriers to defeat Task Force 58. The IJN expected its carrier fleet could defeat TF 58, and the U.S. invasion fleet, with further air support from land-based airfields. However, the Allied aerial bombardment of airfields in the Marianas left the IJN short of land-based air support.
The Battle of the Philippine Sea was one of the campaign's opening battles that emerged shortly after the U.S. Marines began landing on Saipan's beaches. The Japanese fleet launched waves of aircraft, which included hundreds of planes, but they could not reach their targets. TF 58's F6Fs, which were better armored and more suitable for dogfights, wiped out the IJN planes from the skies.
U.S. submarines also intercepted the Japanese aircraft carriers. They fired a spread of torpedoes at two of them, including the fleet's flagship. Explosions aboard the aircraft carriers set them ablaze and their crews abandoned ship.
When contact with TF 58 was briefly lost, Japan's fleet had already lost two aircraft carriers and much of the IJN's fleet air arm. The USS Enterprise later rediscovered another carrier. It launched hundreds more planes against what remained of the IJN fleet. During the bombardment, they wiped out another Japanese fleet aircraft carrier before the IJN warships retreated from the battle. The crushing defeat at the Battle of the Philippine Sea left Japan's garrison at Saipan without any resupply or potential reinforcements.
Having landed ashore Saipan's beaches shortly before the Battle of the Philippine Sea, U.S. Marines consolidated their positions after establishing a beachhead along the southwest coastline. The 27th Army Division landed behind to reinforce the Marines. Upon disembarking on the beaches, it advanced towards the southern airfield at Aslito, which was later abandoned by the Japanese.
When the beaches were lost, the Japanese army retreated to more mountainous terrain. In the center of Saipan there is Mount Tapochau that rises some 1,554 feet. After retreating to the high ground, they set up new positions within the caves and along the cliffs of the mountains. The 27th Infantry Division advanced up the center of Mount Tapochau with two Marine divisions on the east and west flank.
Japanese troops slowed the 27th Division as it 'advanced' up Mount Tapochau. The Marines advanced further on the east and west flanks, which left them with a bent, or U-shaped, line. However, the Japan's grip on the mountain could not be held; and its troops eventually retreated to the northern tip of Saipan. The U.S. troops secured the mountain, and cleared out remaining Japanese divisions in the north around Marpi Point.
In the final days of the battle U.S. Marines captured Garapan on the western coast, or at least what was left of the ravaged village. Naval and artillery shells had reduced Garapan to rubble. Elsewhere they secured Flores Point, and advanced up the beach. The 27th Division continued its advance towards Tanapag Harbor.
Once the Battle of Saipan was over, the Mariana and Palau Islands campaign had almost been won. Even though there were more islands to take in the Marianas and Palau, U.S. troops had secured vital airfields at Saipan. From there, the Air Force could launch bombing raids over nearby Japanese territory, and even the home islands, with its long-range bombers. As the IJN's fleet air arm had also been crushed, the remaining Palau and Mariana Islands had little to no air support for further battles at Guam, Peleliu, etc.
The Japanese Empire was crumbling. After the opening battles, the Allies took further Mariana and Palau Islands to finish Operation Forager. Thereafter, they stepped up their bombing campaigns over the home islands.