The Norman-French military call when, for example, a sentinel spied an enemy force approaching, was “As armes! as armes!”
That summons was introduced into England where it was used for a while, but eventually it was translated into the equivalent English, “at arms!” which became the modern “to arms!”
Similar calls were employed at the same period elsewhere in Europe. That used in Italian armies was “all’ arme! (to arms! )” This became the popular call among other armies; but in every case the words that were called soon became the name of the cry or the name of any kind of signal to indicate danger.
Italian all’ acme! (to arms! ) becanie allarme, French alarme, and English alarm, meaning “a warning sound,” and lost its strictly military use.
The word alarum arose from mispronunciation of alarm, for the same reason that causes many people to sound “film” as if it were spelled “fillum.”