From Longman Dictionary of Contemporary Englishon (the) alert (for something/somebody)on (the) alert (for something/somebody)READYready to notice and deal with a situation or problem Be on the alert for anyone acting suspiciously. Troops in the vicinity were put on alert.on full alert (also on high alert) (=completely ready to deal with a dangerous situation) All our border points are on full alert. → alert
Examples from the Corpus
on high alert• Early in the war Soviet southern troops were put on high alert. 