South Vietnam's 21st Division, decimated by repeated attempts to relieve An Loc, is replaced by the 25th Division. At the same time, U.S. helicopters flew 18th Division troops to positions south of An Loc to replace badly battered 9th Division troops that had also been trying to get to the city.
The 21st Division and attached units had been trying to reach the besieged city since April 9, when the group had been moved from its normal station in the Mekong Delta and ordered to attack up Highway 13 from Lai Khe to open the route to An Loc. The South Vietnamese forces had been locked in a desperate battle with a North Vietnamese division blocking the highway since the very beginning of the siege. As the 21st Division tried to open the road, the defenders inside An Loc fought off repeated attacks by two North Vietnamese divisions that had surrounded the city early in April. This was the southernmost thrust of the North Vietnamese invasion that had begun on March 30; the other main objectives were Quang Tri in the north and Kontum in the Central Highlands.
The arrival of the fresh South Vietnamese soldiers would eventually result in the lifting of the siege at An Loc. The 18th Division troops successfully attacked the North Vietnamese forces surrounding the city and most of the communist troops within An Loc had been eliminated by the end of the month. The 25th Division was less successful and the North Vietnamese forces continued to block Route 13 south of the city.