be on (one's) guard

To be especially careful or vigilant; to be prepared for danger or surprises. He's got a lot of tricky moves, so be on your guard! Those trails can be a bit treacherous to climb. I just hope she's on her guard. When you're in the big city, you need to be on your guard against criminals.

be on guard

To be very attentive and alert, so as to avoid danger in a particular situation. A possessive adjective can be used between "on" and "guard." When you're in the big city, you need to be on guard against criminals. I'm on my guard because I know the older guys on the team are going to prank me. I hope she's on her guard. Those trails can be a bit treacherous to climb.