"Havoc" has the sense of being "out of control" or "occurring spontaneously" or "as a by-product of something else". So it is out of context to "order" it as a general in an army. The general could order an "attack" and then if the soldiers went out of control, they would wreak havoc. You don't plan "havoc" it just happens! Havoc is something a general would actually try to avoid since it would indicate that he had lost control of his men.
Hope this helps.