Complex sentence: Not only did North Vietnam conquer South Vietnam but the communist were also victorious in Cambodia when the Khmer Rouge captured Phnom Penh on April 13, as were the Pathet Lao successful in capturing Vientiane on December 2.
Split into simpler sentences: Not only did North Vietnam conquer South Vietnam but the communist were also victorious in Cambodia when the Khmer Rouge captured Phnom Penh on April 13. As were the Pathet Lao successful in capturing Vientiane on December 2.