"Havoc" was a command in the British Navy of Shakespeare's time period. After defeating the standing army of a feudal lord, village or city, if the locals were still aggressive or quartering AWOL enemy soldiers, the command of "Reap havoc!" or just "Havoc!" was issued. This allowed the soldiers to pillage the town or castle.
Cry (yell) 'Havoc,' and let slip (release) the dogs of war (soldiers);
Cry (yell) 'Havoc,' and let slip (release) the dogs of war (soldiers);