From Italian all'arme (“to arms”), through Old French alarme . The variant alarum likely arose from phonetic spelling and Latinising tendencies in Early Modern English printing.
By the 18th century, alarm became standard. Alarum survived mainly in archaising poetry or legal phrases (“alarum and excursion”), but faded from common use. alarum download
In theatre, an alarum signalled an off-stage battle or urgent call to arms, often accompanied by drumming or trumpets. It conveyed chaos and urgency without staging full combat. From Italian all'arme (“to arms”), through Old French