Ahab is developed slowly throughout the book, given that he doesn't even appear physically until around the middle chapters. He begins as a force of nature, Melville describes him as being indomitable, as being willing to do anything to seek his revenge on the creature. The reader at first sees this as his great power, though there are hints of the somewhat more broken character of the man.
As the plot progresses and Ahab begins to descend into more of a madness, it becomes clear that as he descends, he is more and more a weak man being driven by an obsession, rather than a powerful man seeking some kind of revenge. He is controlled rather than controlling, representing somewhat of a reversal of his role. In actuality this was always the case, but Melville takes his time revealing this.