Alarm, Tocsin and Alert all agree in meaning a signal that serves as a call to action or to be on guard especially in a time of imminent danger.
Alarm is used of any signal that arouses to activity not only troops, but emergency workers (as firemen, policemen); it suggests a sound such as a cry, a pealing of a bell, a beating of drums, or a siren.
- sound a fire alarm
- the dog’s barking gave the alarm
Tocsin may be either an alarm sounded by bells usually from the belfry of a church or, more often, the bells sounding an alarm. but is used figuratively for any sort of warning of danger.
- the loud tocsin tolled their last alarm
—Campbell
Alert, a military term for a signal to be on guard and ready for expected enemy action, is often used for any warning of danger.
- sirens sounded an air-raid alert
- the Weather Bureau issued a tornado alert in the early afternoon . . . . The alert was cancelled after 5 p.m.
—Springfield Union
It may also denote the state of readiness called for by the signal or warning or the period during which this is maintained.
- an abandon-ship alert was signaled. Warning for that . . . was to be four blasts of the ships whistle
—Lowell Bennett - a peacetime round-theclock alert against surprise aerial attack
—N. Y. Times