"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);
Caesar uses the phrase "cry havoc" in many of his plays
In Julius Caesar, Mark Antony says this after Caesar has been killed. He is saying there is a war coming along with panic and confusion
This passage is referring to going to war. When ceasar thought that he a god and tried to take over all counties, there was a country in particular that stood up to ceaser so this is what was said to ceaser's men.
This quote is from Richard II, not the play but the King himself in the 9th year of his reign. This was from a set of ordinances to his army. Of course the word Havoc meaning general destruction or devastation you get the idea.
X. ITEM, that no one be so hardy as to cry "havok," under pain of losing his head, and that he or they that shall be the beginners of the said cry shall likewise be beheaded, and their bodies afterwards be hanged up by the arms.