Definitions for "Red Alert"
Red was the British Civil Defence action warning 'Raiders Approach', receipt of which would authorise the sounding of the audible sirens on a two minute fluctuating or warbling note of varying pitch giving 12 minutes advance warning of the enemy's arrival.
the highest level of alert when an attack by the enemy seems immanent (or more generally a state of alert resulting from immanent danger)
a warning that an enemy nuclear attack is imminent or already in progress.