on guard
on one's guard
upon one's guard
on your guard

What was more, he would be especially on his guard now.

Women in public life still have to be on their guard.

No one could be on his guard all the time.

However, so long as we were on our guard, nothing could happen to us.

I must be on my guard while you are in the house.

They may not know, and any action we'd take now would put them on their guard.

She was going to have to be on her guard.

And even if he did, she was now on her guard.

At the same time, he must be on his guard.

At the moment he was certainly not on his guard.