Well, in Shakespeare's Julius Caesar, "Cry 'Havoc!' and let slip the dogs of war," is part of a soliloquy from Antony's perspective. Caesar has just been assassinated, and Antony is predicting the horrors that are going to grip Italy now that Caesar is dead. Antony talks about "fierce civil strife" that will take over Italy. By this, he's talking about civil war. Now that Caesar, the leader, is dead, no one knows who's going to be able to claim power. Antony thinks that Italy is going to be plunged into a brutal, awful war, and that Caesar's ghost is going to watch everything happen and feel vindicated by it. For Caesar, this massive war will be revenge for his murder. So, as a ghost, Caesar will "cry havoc," or declare a state of chaos, and "let slip" (release) the "dogs of war" (the different factions that will go to war over Italy).
Hope this helps! :)