We say an enemy on the move is easy to attack precisely because he is then off guard, that is, unprepared.

我们说运动之敌好打，就是因为敌在不意即无准备中。

youdao

They are more likely to be caught off guard, unaware, unprepared and put in a position to act on something they had not enough time or information about to consider a reasonable approach.

FORBES: Networking- Have you made it part of your job?