 To further enhance their political posture at the Paris talks , which opened on 13 May , the North Vietnamese opened the second phase of the General Offensive in late April . U.S. intelligence sources estimated between February and May the North Vietnamese dispatched 50 @,@ 000 men down the Ho Chi Minh Trail to replace losses incurred during the earlier fighting . Some of the most prolonged and vicious combat of the war opened on 29 April and lasted until 30 May when the 8 @,@ 000 men of the 320th PAVN Division , backed by artillery from across the DMZ , threatened the U.S. logistical base at Đông Hà , in northwestern Quảng Trị Province . In what became known as the Battle of Dai Do , the North Vietnamese clashed savagely with U.S. Marine , Army , and ARVN forces before withdrawing . The North Vietnamese lost an estimated 2 @,@ 100 men after inflicting casualties on the allies of 290 killed and 946 wounded . 
