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 PAVN 320th 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 PAVN clashed savagely with U.S. Marine, Army and ARVN forces before withdrawing. The PAVN lost an estimated 2,100 men according to US/ARVN claims, after inflicting casualties on the allies of 290 killed and 946 wounded.