Chances are, it was in an English class!
The phrase is actually a truncated version of a famous line from Shaespeare's play Julius Caesar, and the complete line goes like this:
"Cry 'Havoc!' And let slip the dogs of war." (Julius Caesar, Act 3, Scene 1, Line 273, to be precise)
|Public Domain from Wikimedia Commons|
But did the idea originate from Shakespeare? And if not, where did he get it from?
According to Shakespearean scholar Jonathan Bate, the Bard used a book written in by a first century historian as his main resource when writing Julius Caesar. A mention of the "war dog," by a Greek historian named Plutarch (ca. 96-98 C.E.) most probably influenced Shakespeare's writing.
As far as the cry, "Havoc!" is concerned? According to the UK's Phrases.org site, that was a signal given by English military to forces in the Middle Ages - to send the troops out to pillage.
The site quotes a text by Thomas De Brotherton, the first Earl of Norfolk (early 1300's) in which he wrote: "...when they be brought into the field and cried havoke, then every man to take his part."
So there you have it.
War dog reference, Plutarch: Life of Aratus, chapter 24. "The Achaeans put a garrison of 400 men in the citadel of Corinth, which was strengthened with 50 dogs, and as many men to keep them."