Complex sentence: By early 1965, the majority of rural South Vietnam was under limited VC control, increasingly supported by NVA regulars from North Vietnam, while ARVN units in the field were hopelessly outclassed and entire units were repeatedly ambushed and slaughtered.
Split into simpler sentences: By early 1965 the majority of rural South Vietnam was under limited VC control, increasingly supported by NVA regulars from North Vietnam. By 1965, ARVN units in the field were hopelessly outclassed and being ambushed and slaughtered.