The Rise of the Viet Cong: Through the horrors of the Vietnam War the time travelers struggled to understand the mindless aggression of the human ... to live or must die. (Extinction's Edge)

With two Timeships hidden in fixed orbit behind Pluto in the farthest reaches of the solar system, the extinction of the human race is imminent. It is only the watchers in the Earth observation capsule who are now pleading for more time and a longer study before the final decision is made. Reluctantly three more mind melts are sanctioned. The first new contact is with Nguyan Nam Kim who is taken as a boy from his home village and indoctrinated in the guerrilla camps in the jungles. Nam Kim grows from a hit and run fighter engaging in ambush and night attacks, to a seasoned veteran of the Viet Cong. The climax comes in the initially victorious battle for Hue, the old imperial capital on the Perfume River, followed by the merciless US counter-attack which drove the Viet Cong out of the city.