撤兵 is a verb-object compound (离合词) meaning 'to withdraw troops'. As a separable verb, the object 兵 can separate from 撤 when adding measure words, specifics, or aspect markers — for example, 撤了兵 ('withdrew troops'), 撤过兵 ('have withdrawn troops'), or 撤了三千兵 ('withdrew three thousand troops'). You can also insert durations or quantities between the two parts.