Divorce can be by death, by mutual consent of both the parties and also by judicial process. According to Islamic law, after the death of wife, husband can marry at any time he likes. On the other hand a wife can marry only after 4 months and 10 days of the death of her husband so that doubts about pregnancy etc. are removed.
Divorce can also be through judicial process. Such a divorce can be on the basis of adultery and impotency and also because off false charges of adultery made by the husband. The system of divorce thus makes it quite clear that the husband has a upper hand over his wife in so far as divorce or talaq is concerned.
It is because whereas the husband can leave his wife any time he likes, the wife finds it very difficult to leave her husband, even if both arc not harmoniously pulling.