 Fourth and last phase started in early 644 with the invasion of Khurasan.After a Decisive Battle of Oxus river, Yazdegerd III fled to central Asia and Persian Empire ceased to exist.Umar's conquest of Sassanid Persian empire by commanding the operations, while sitting about 1000 kilometer away from the battle fields, will become the greatest triumph of Umar and his strategic marvel, and marked his reputation as one of the greatest military and political genius of history, like his late cousin Khalid ibn Walid (590 – 642).