1540s, from Middle French cavalerie (16c.), from Italian cavalleria "mounted militia," from cavaliere (see cavalier (n.)). An Old English word for it was horshere.
Last-minute rescue forces; a deus ex machina: Powell likened the Somalia operation to ''the cavalry coming to the rescue''/ not the time for a President who avoided the draft to call up the cavalry/ Democrats hear hoofbeats—the cavalry's arrival, in the nick of time
[1980s+; fr the numerous cases in cowboy movies when the US Cavalry would arrive to rescue various beleaguered persons]