The troops were sent in for mopping-up operations to clear the remaining enemy resistance.

mopping-up 是由动词短语 mop up（彻底清除）加上连字符和-ing 构成的复合名词。mop 本意是"拖把"，用作动词表示"擦除、清除"，up 表示"彻底、完全"。 mopping-up operations 指军事行动中彻底清除残余敌人的"扫荡行动"。