Otto II was made joint-ruler of Germany in 961, at an early age, and his father named him co-Emperor in 967 to secure his succession to the throne. His father also arranged for Otto II to marry the Byzantine Princess Theophanu, who would be his wife until his death. When his father died after a 37-year reign, the eighteen-year-old Otto II became absolute ruler of the Holy Roman Empire in a peaceful succession. Otto II spent his reign continuing his father's policy of strengthening Imperial rule in Germany and extending the borders of the Empire deeper into southern Italy. Otto II also continued the work of Otto I in subordinating the Catholic Church to Imperial control.