On 23 August 1967, the North Vietnamese Air Force employed their newly devised tactic against a U.S. strike formation, while it was conducting raids against a rail yard.

[2] In the first phase of the operation, which lasted from spring to summer of 1965, American airpower mainly targeted the North Vietnamese capital of Hanoi and its limited industrial base.

[3] For the third phase of Rolling Thunder, which occurred from spring 1967 to early 1968, bombing operations were focused on industrial and transportation targets in and around Hanoi, Hai Phong and the buffer zone near the Chinese border.

[4] The third phase of the campaign was heavily favored by U.S. commanders, because it allowed them to destroy rather than just threaten Hanoi's nascent industrial infrastructures.

On 2 January 1967, a total of 56 F-4C Phantom II from the 8th and 366th Tactical Fighter Wings flew in a formation which resembled an F-105 strike force, towards the VPAF's Phuc Yen Air Base.

The actions of 2 January had exposed the flawed tactics employed by MiG-21 pilots; they broke through the clouds too quickly, and they did not join up with each other before they attacked the U.S. fighters waiting above them.

Before the North Vietnamese Air Force could implement their new tactics, however, the VPAF 921st Fighter Regiment was withdrawn from combat for several months to recover from the bloody defeat it had suffered as a result of Operation Bolo.

As a result, between late June and early August 1967, North Vietnamese Air Force fighters were grounded several times, so their pilots could work on their training and new tactics.

[11] To minimize the effects of U.S. strikes, North Vietnamese authorities constructed several alternate rail yards on the north-eastern train line, including the Yen Vien rail yard which located 58 kilometers (36 mi) north of Hanoi and ran through Thai Nguyen, and linked up with the main line at Kep close to China.

[12] In response, the Seventh Air Force launched repeated strikes on the Yen Vien rail yard and other infrastructures which linked Hanoi with the China buffer zone.

The Americans claimed to have damaged more than half of the boxcars and trapped the remainder in the rail yard, so they returned two days later to destroy the rest.

The mission was led by Colonel Nicholas J. Donelson with Olds, who spearheaded Operation Bolo back in January, as the leader of the escort formation.

[13] At 1:45 pm on 23 August, North Vietnamese radars detected a flight of 40 U.S. aircraft approaching Hanoi from Sam Neua, in neighboring Laos.

After Chieu and Coc had departed from Phuc Yen Air Base, they flew at low level and stayed in the ground clutter to avoid detection by airborne radars employed by U.S. aircraft.

However, the MiG-21's transponders were detected by an orbiting EC-121 airborne early warning radar surveillance aircraft, and their positions were relayed to the F-4 fighters that were escorting the strike formation, but they did not respond.

[17] In contrast to their opponents, the action of 23 August gave the North Vietnamese Air Force their first major victory since Operation Bolo.

The USAF's confirmation of the loss of 'Ford 4' to a MiG aircraft gave Nguyen Van Coc his second air-to-air victory, and he eventually became the leading ace pilot of the war with nine kills.

[16] Following their defeat at the hands of the North Vietnamese Air Force, Colonel Robin Olds learned that Seventh Air Force intelligence had watched North Vietnamese MiG fighters practicing their new tactics for ten days prior to the battle of 23 August, but had not passed that information on to the 8th Tactical Fighter Wing and other units.

[13] In the first half of 1967, the North Vietnamese had realized they could not directly confront U.S. fighters in air-to-air combat, so they changed their procedures for the deployment of their numerically inferior MiG units.

The action of 23 August demonstrated that the North Vietnamese Air Force had successfully executed their new procedure, which was helped by more skilful ground controllers who directed the MiG fighter towards their targets.