Fifth War Patrol: Philippine Mission
Even greater successes lay ahead. Having sunk one destroyer, Harder joined the all-out hunt against Japanese destroyers, once considered the most dangerous of foes. Assigned the area around the Japanese fleet anchorage at Tawi-Tawi, Harder departed Fremantle on 26 May 1944 with Redfin (SS-272) and headed for the Celebes Sea.
On 6 June Harder entered the heavily patrolled Sibutu Passage between Tawi-Tawi and North Borneo and encountered a convoy of three tankers and two destroyers. She gave chase on the surface but was illuminated by the moon. As one of the destroyers turned to attack, Harder submerged, turned her stern to the charging destroyer, and fired three torpedoes at range of 1,100 yards (1,000 m). Two struck Minazuki and exploded; the destroyer sank within five minutes. After attacking the second escort without success, Harder was held down by a depth charge attack while the convoy escaped.
Early next morning an enemy plane spotted Harder. The submarine soon sighted another destroyer searching the area for her. As before, Harder took the initiative as the enemy closed the range. The sub fired three torpedoes at short range, and two of them struck amidships, one detonating the ship's magazine with a tremendous explosion. Hayanami sank a minute later. Following the inevitable depth charge attack, Harder transited the Sibutu Passage after dark and steamed to the northeast coast of Borneo. There on the night of 8 June she picked up six British coastwatchers, and early next day she headed once more for Sibutu Passage.
That evening Harder sighted two enemy destroyers patrolling the narrowest part of the passage, just a miles from Tawi-Tawi. After submerging, she made an undetected approach and at 1,000 yards (900 m) fired four torpedoes at the overlapping targets. The second and third torpedoes blasted Tanikaze; she sank almost immediately, her boilers erupting with a terrific explosion. The fourth shot hit the second ship and exploded with a blinding flash. Within minutes Harder surfaced to survey the results, but both ships had disappeared. Soon afterward, she underwent the inevitable depth charge attack by enemy planes, then she set course for a point south of Tawi-Tawi to reconnoiter.
On the afternoon of 10 June Harder sighted a large Japanese task force, including three battleships and four cruisers with screening destroyers. An overhead plane spotted the submarine at periscope depth and a screening escort promptly steamed at 35 knots (65 km/h) toward her position. Once again, Harder became the aggressive adversary. As the range closed to 1,500 yards (1,400 m), she fired three torpedoes on a "down the throat" shot, then went deep to escape the onrushing destroyer and certain depth charge attack. Within a minute two torpedoes blasted the ship with violent force just as Harder passed her some 80 feet (24 m) below. The deafening explosions shook the submarine far worse than the depth charges and aerial bombs which the infuriated enemy dropped during the next two hours. When she surfaced, Harder saw only a lighted buoy marking the spot where the unidentified destroyer either sank or was heavily damaged.
Harder reconnoitered Tawi-Tawi anchorage 11 June and sighted additional enemy cruisers and destroyers. At 16:00 she headed for the open sea and that night transmitted her observations which were of vital importance to Admiral Raymond A. Spruance's fleet prior to the decisive Battle of the Philippine Sea. Harder steamed to Darwin on 21 June for additional torpedoes, and, after patrolling the Flores Sea south of the Celebes Islands (with Admiral Ralph Christie aboard), she ended the patrol at Darwin on 3 July.
The important results of Harder's fifth war patrol have caused some to call it the most brilliant of the war. Not only did Harder further deplete the critical supply of destroyers by sinking four of them and heavily damaging or destroying another one in four days, but her frequent attacks and a rash of enemy contact reports on this fleeting marauder so frightened Admiral Soemu Toyoda that he believed Tawi-Tawi surrounded by submarines. As a result, Admiral Jisaburo Ozawa's Mobile Fleet departed Tawi-Tawi a day ahead of schedule. The premature departure upset the Japanese battle plans, and forced Ozawa to delay his carrier force in the Philippine Sea, thus contributing to the stunning defeat suffered by the Japanese in the ensuing battle.