At the beginning of the war, complete units of «Zenith» missile-launchers departed from the USSR to Viet Nam. The 2nd Briansk battalion, for example, left with all its members. There we tested the new material, improved it to escape from the missiles and learned how to shoot against small targets. We only used surface-to-air C-75 launching devises, because the first C-125 appeared in Egypt. The Vietnamese wanted the C-125, but that implied that the material had to go through China and we feared that part of it did not arrive at its destination. I arrived after flying over Afghanistan, Pakistan, India and Burma. There was no other way. The agreements with those countries were valid in terms of people, but not in terms of arms, and the American ships made it dangerous to send the material by sea. The Vietnamese received the C-125 but the war had ended. They also wanted the missile-launcher «Strela-1», but we did not want its technology to be disclosed. _ We learned from the beginning that the Americans were not prepared for that war. Their planes flew at a height that made them easy targets for us, and we did not have any tactical information. The Major Proskurnin was decorated for hitting four targets with only three missiles! The United States had underestimated our means and was shocked after it suffered serious losses. The US planes started to fly lower and became a good target for the Vietnamese antiaircraft artillery. In 1972 the Americans triggered the «Linebacker 2» operation, when the B-52 bombed the whole northern part of the country. They later understood that there were no targets to attack there, since the weapons were hidden in the mountains. Napalm was used in the South to force combatants out of the jungle. The Vietnamese did not authorize the Russian specialists to participate in the military decision-making, but we tried to gather information about the operations in the South, and to know when they were going to expand to Cambodia or Laos. It was impossible. The Party was on high alert.

Source
Vremya Novostyey (Russia)

Вьетнамцы старались, чтобы мы не командовали”, par Evgueni Antonov, Vremya Novostyey. This text has been adapted from an interview.