The first F-4C Phantoms of the USAF arrived in Vietnam at the same time as those of the Marines, in April 1965, with the 45th Tactical Fighter Squadron, followed by those of the 12th TFW, in November, and of the 8th TFW, which was stationed at the Ubon base in Thailand in December of the same year. The Phantoms were detailed to play a defensive fighter role, escorting the F-105s weighted down with their bomb-loads, but when the ranks of the latter began to thin out, the Phantoms also took on attacking roles, achieving excellent results with their precision bombing. Although many units covered themselves with glory in eight years of war, it was the 8th TFW, among the first to reach Vietnam, which was most highly distinguished in battle. On January 2, 1967, F-4Cs of this Wing played a key role in the biggest aerial encounter of the war, shooting down seven MiG-21s without loss; and the 555th TFS (which with the 432nd and 433rd formed the 8th TFW) achieved more victories than any other USAF squadron, with a tally of 39 MiGs. In such dogfights the radar-controlled Sparrow air-to-air missiles and heat-sensitive Sidewinders proved invaluable, but the lack of a traditional cannon, notably for strike missions, soon became evident. It was for this reason that the F-4E version, sent into action toward the end of the war, was equipped with a rotary 20mm cannon, soon proving its worth not only in hitting the enemy on the ground but also in shooting down six enemy jets. By the end of the war the F-4s of the USAF boasted a record of 82 victories in air duels with MiGs, the success ratio in favor of the Phantom pilots being more than two to one.