During the next few years of his career, LCDR Thach's superior performance brought him to be assigned as a test pilot and flight instructor and to receive a Letter of Commendation in 1940 "for exceptional skill and technique in aerial gunnery and bombing; efficient and meticulous operation of a squadron gunnery department, and marked ability to train other pilots in fighting plane tactics and gunnery."

When the United States entered World War II, LCDR Thach was the Commanding Officer of Fighting Three embarked on the carrier Saratoga. At the time Admiral Thach was one of the top fighter tacticians in the Navy. Intelligence reports from the Sino-Japanese war convinced him that the Navy's top carrier fighter, the F4F Grumman Wildcat, was no match for the superior flying performance of the Japanese A6M Zero. While stationed on the West Coast after Saratoga was torpedoed in January 1942, LCDR Thach used match sticks on his kitchen table to revise the standard fighter formation from two-three plane sections to two-two plane sections. In addition, the two sections were deployed abreast of each other at a distance equal to the turning radius of the fighters. This allowed the attacked section to turn towards the other section, setting up the enemy fighters to be driven off by the other section.

LCDR Thach took Fighting Three on Lexington in the spring of 1942, and participated in the aborted attack on Rabaul in February and the attack on Lae-Salamua in March. After these attacks, Fighting Three went ashore on Oahu until late May 1942.

This new tactic, called the 'Beam Defense Position', but dubbed the 'Thach Weave' by LCDR James H. Flatley, was first tested by LCDR Thach and Fighting Three while embarked on Yorktown at Midway in June 1942. LCDR Thach (flying F-23, BuNo 5093) and five other pilots from Fighting Three were charged with escorting Torpedo Three in their attack on Kido Butai on the morning of June 4. After losing a fighter early in the fight, LCDR Thach initiated his "Beam Defense Position." Despite being outnumbered, LCDR Thach was credited with the destruction of three Zero's, with his wingman, ENS Robert A. M. Dibb, also shooting down a Zero.

Later on June 4, LCDR Thach was credited with the destruction of a B5N2 Kate torpedo bomber, probably flown by LT Joichi Tomonaga, commander of Hiryu's air group, during the second Japanese attack on Yorktown.

LCDR Thach reported to Pearl Harbor after Midway to instruct other pilots in the use of his new technique. Later in the war LCDR Thach was assigned to Fast Carrier Task Force as Air Operations Officer where he developed the system of blanketing enemy airfields with a continuous patrol of carrier based fighters that was credited with destroying the air offensive capabilities of Japan.

LCDR Thach continued his distinguished career after the Second World War. He commanded the carrier Sicily during the Korean conflict and later Franklin D. Roosevelt. In addition to these commands, LCDR Thach was also assigned to various high level staff billets. He was promoted to the rank of RADM in November 1955, VADM in January 1960, and ADM in March 1965. ADM Thach became commander of the Anti-Submarine Warfare Force of the Pacific Fleet in 1960, and in recognition of his work the Navy annually awards the best Anti-Submarine Warfare Squadron "The Admiral Thach Award." In 1965 ADM Thach was ordered to duty as Commander-in-Chief of U.S. Naval Forces in Europe and served there until his retirement in May 1967, after more than 40 years service.

Credited with 6.0 victories during the war. ADM Thach participated in twelve major engagements or campaigns and was awarded the following distinctions:

Distinguished Service Medal, Navy Cross, Letter of Commendation from ADM Nimitz, Gold Star in lieu of Second Navy Cross, Silver Star Medal, Legion of Merit, Bronze Star Medal, Gold Star in lieu of Second Legion of Merit, plus other various campaign, service and unit awards.

ADM John Smith Thach died on 15 April 1981.