sentences:  This move would force the PAVN to reinforce and resupply across the Thạch Hãn River, making them vulnerable to air strikes.The helicopters were met by heavy anti-aircraft fire with one CH-53 hit by an SA-7 and crashing with 2 U.S. Marine crewmen and 45 Vietnamese Marines killed.Two CH-46s were shot down while another 25 helicopters were damaged.