on the alert

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See: careful
References in classic literature ?
They had to be continually on the alert, too, against the mountain tribes, who beset every defile, laid ambuscades in their path, or attacked them in their night encampments; so that, of the hardy bands of trappers that first entered into these regions, three-fifths are said to have fallen by the hands of savage foes.
Kennedy was on the alert for her, and his bullet broke her shoulder.