What does "cry havoc" mean?

Sound an alarm or warning, as in In his sermon the pastor cried havoc to the congregation's biases against gays. The noun havoc was once a command for invaders to begin looting and killing the defenders' town. Shakespeare so used it in Julius Caesar (3:1): “Cry 'Havoc' and let slip the dogs of war.” By the 19th century the phrase had acquired its present meaning.