In March 1965, U.S. air strikes into North Vietnam began. In April, the first dogfights took place. As the fighting grew into a major conflict, the air activity increased accordingly. At first the results were not encouraging for the U. S. Air Force. The most common problem found could be summed up in the words insufficient training and experience in air-to-air combat. But in 1972, when the "Top Gun" program improved the skills in aerial combat of USN Phantom pilots, and the F-4E appeared with a 20 mm built-in Vulcan cannon, could the US neutralize edge of the NVAF MiG-17, MiG-19 and MiG-21.