A strong veteran, having trained for tens of years, can fall to a weakling in a moment of laxness. That's what the martial arts world is.

It could be a trap; don't let your guard down.

That you should be stalking Hanako! You'd been quiet recently so I'd let my guard down.

Security is the greatest enemy.

Plenty of opportunities will present themselves, if only you are awake to them.

You'd better stay on your guard.

I was on the alert for a fugitive criminal.

Be alert when you cross a busy street!

Accidents will happen when we are off guard.

I realized I was off my guard.

We must be alert to dangers.

My watchdog is alert to the slightest sound and movement.

You can't let your guard down for a second around that guy.

You must be careful when you drive a car.

You've got to be on your toes.