on the alert

警戒,处于戒备状态

on foot, on guard, on hand, on instalments, on instinct, on no account, on occasion, on one's guard, on one's head, on one's honor, on one's honour, on one's own, on one's way to..., on order that, on principle, on purpose, on relief, on sale, on schedule, on second thoughts, on the analogy of, on the average, on the basis of, on the contrary, on the eve of..., on the first try, on the ground that, on the grounds of, on the impulse (of...), on the increase, on the lips of, on the nail, on the other hand, on the part of, on the point of, on the principle of, on the quiet, on the road, on the run, on the side,