During bombing missions, unescorted U.S. Republic F-105 Thunderchief fighter-bombers would often be attacked by MiG-21s. Heavily laden with ordnance, the F-105s were no match for the faster MiGs. On occasions when the F-105s were escorted by F-4 Phantom II fighter planes, the MiGs would refuse to engage. In addition, the rules of engagement on the U.S. side prevented attacking the MiGs when they were on the ground. This measure was intended to keep the Soviet Union from becoming further engaged in the conflict by preventing casualties to the Soviet mechanics and technical advisors who were assisting North Vietnam.