 Smith was held prisoner until released on 15 March 1973, due to improving US-China relations following U.S. President Richard Nixon's visit to China in 1972.The Vietnam People's Air Force (VPAF) began receiving the MiG-19 at the end of Operation Rolling Thunder, which ended in 1968.Despite their limited numbers, MiG-19s were involved in extensive combat during Operations Linebacker and Linebacker 2.