Cry havoc and let slip the dogs of war
The military order Havoc! was a signal given to the English military forces in the Middle Ages to direct the soldiery (in Shakespeare's parlance 'the dogs of war') to pillage and chaos.
The Black Book of the Admiralty, 1385 is a collection of laws in French and Latin that relate to the organisation of the English Navy. In the 'Ordinances of War of Richard II' in that book we find:
"Item, qe nul soit si hardy de crier havok sur peine davoir la test coupe."
An English text which comes nearer to defining the term is found in Grose's Military Antiquities Respecting a History of the English Army, 1801. Grose was quoting a translation of an Old French text by Thomas De Brotherton, the first Earl of Norfolk (Brotherton died in 1338):
"Likewise be all manner of beasts, when they be brought into the field and cried havoke, then every man to take his part."
Shakespeare was well aware of the use of the meaning of havoc and he used 'cry havoc' in several of his plays. The 'cry havoc, and let slip the dogs of war' form of the phrase is from his Julius Caesar, 1601. After Caesar's murder Anthony regrets the course he has taken and predicts that war is sure to follow.
Blood and destruction shall be so in use
And dreadful objects so familiar
That mothers shall but smile when they behold
Their infants quarter'd with the hands of war;
All pity choked with custom of fell deeds:
And Caesar's spirit, ranging for revenge,
With Ate by his side come hot from hell,
Shall in these confines with a monarch's voice
Cry 'Havoc,' and let slip the dogs of war;
That this foul deed shall smell above the earth
With carrion men, groaning for burial.
The term is the predecessord of 'play havoc' (with). This is now more common than 'cry havoc' but has lost the force of the earlier phrase - just meaning 'cause disorder and confusion'.
See other - phrases and sayings from Shakespeare.