 Westmoreland was more concerned with the situation at Khe Sanh , where , on 21 January , a force estimated at 20 @,@ 000 – 40 @,@ 000 North Vietnamese troops had besieged the U.S. Marine garrison . MACV was convinced that the North Vietnamese planned to stage an attack and overrun the base as a prelude to an all @-@ out effort to seize the two northernmost provinces of South Vietnam . To deter any such possibility , he deployed 250 @,@ 000 men , including half of MACV 's U.S. maneuver battalions , to the I Corps Tactical Zone . 
