The new destroyer escort conducted shakedown training off Bermuda before departing Hampton Roads 5 June 1944 for the Pacific. Sailing via the Panama Canal, she arrived Pearl Harbor 26 June and engaged in convoy and training operations during July. John C. Butler then departed Pearl Harbor 9 August screening transports bound for the invasion of the Palaus. After seeing them safely to Tulagi, the ship operated with escort carriers out of Manus on preinvasion strikes. Two islands wanted as advance bases for the long-awaited move into the Philippines, Morotai and Peliliu, were stormed 15 September; and John C. Butler provided antisubmarine and antiaircraft protection for the supporting carriers. Returning to Manus 30 September, she replenished in preparation for the Leyte operation in October.
The escort vessel sailed with Rear Admiral Oftsie's escort carrier group 12 October to provide air cover for the massive movement of transports into Leyte Gulf. After the initial landings, the three carrier groups, soon to become famous by their radio code names, 'Taffy 1,' 'Taffy 2,' and 'Taffy 3,' took station east of the Philippines to lend close air support.
The Japanese fleet was closing the Philippines in a last attempt to annihilate the invasion force, with heavy ships designated to break into Leyte Gulf from north and south, and a diversionary fleet of carriers to draw Halsey off to the North. In the first two actions of the massive Battle for Leyte Gulf which ensued, the Battles of Sibuyan Sea and Surigao Strait, the Japanese were badly mauled. But Admiral Kurita's Center Force still transited San Bernardino Strait the night of 24-25 October and just after sunrise bore down on the relatively unprotected 'Taffy 3,' including John C. Butler.
The 2-hour battle off Samar which followed has taken a rightful place among the most memorable actions in naval history. The slow escort carriers launched all planes to attack the Japanese cruisers and battleships, and John C. Butler and her sisters laid heavy smoke to confuse enemy batteries. A rain squall provided cover for a turn to the south, and just after 0730 the destroyers began their gallant torpedo attacks against great odds. Johnston, Hoel, Heermann, and escort Samuel B. Roberts made close-in attacks on cruisers and battleships, forcing them to zig-zag, while aircraft made continuous attacks. Soon after this first attack, John C. Butler turned from the carriers to launch her remaining torpedoes. then exchanged gunfire with a heavy cruiser. The destroyer escort continued to fire and dodge heavy-caliber fire until dangerously low on ammunition, then returned to the carrier formation to provide smoke coverage. Admiral C.A.F. Sprague, commander of Taffy 3, later described the next surprising development: :At 0925 my mind was occupied with dodging torpedoes when near the bridge I heard one of the signalmen yell, '. . . dammit, boys, they're getting away!' I could not believe my eyes, but it looked as if the whole Japanese fleet was indeed retiring. . . . At best, I had expected to be swimming by this time.' The Japanese, damaged and fearing heavier air attack, had indeed reversed course. Though the escort carriers lost two of their numbe