Drop (one's) guard phrase
The verbs "lower" and "let down" can be substituted for the verb "drop".
To stop being guarded, cautious, or alert about potential danger or difficulty.
Businesses don't want to drop their guard when it comes to investment.
It takes her quite a long time to drop her guard and open up her heart.
The verb "drop" should be conjugated according to its tense.
The expressions "drop one’s guard", in a figurative sense, seem to have only become popular in the mid-twentieth century, though the phrases have been in use in a literal sense for many years before that.
Used to express something bad is happening again
Here we go again! Lisa has just asked to borrow my new dress.