II's older brothers Henry (born 952) and Bruno (born 953) had died, as well as Otto I's son from his first wife Eadgyth, the Crown Prince Liudolf, Duke of Swabia. With his older brothers dead, the two-year-old Otto became the Kingdom's crown prince and Otto I's heir apparent. Otto I entrusted his illegitimate son, Archbishop William of Mainz, with Otto II's literary and cultural education. Margrave Odo, commander of the Eastern March, taught the young crown prince the art of war and the kingdom's legal customs. Needing to put his affairs in order prior to his descent into Italy, Otto I summoned a Diet at Worms and had Otto II elected, at the age of six, co-regent in May 961. Otto II was later crowned by his uncle Bruno the Great, Archbishop of Cologne, at Aachen Cathedral, probably on Pentecost (26 May). While Otto I had secured succession of the throne, he had violated the Kingdom's unwritten law that succession rights could only be granted to a child who has reached the age of majority. He was likely motivated by the high risk associated with his expedition into Italy to claim the Imperial title from the Pope. Otto I crossed the Alps into Italy, while Otto II remained in Germany, and the two archbishops, Bruno and William, were appointed as his regents. After three-and-a-half years in Italy, Otto I returned to Germany early in 965 as Holy Roman Emperor. In order to give the hope of dynastic continuity after his death, Otto I again confirmed Otto II as his heir on 2 February 965, the third anniversary of Otto I's coronation as Emperor. Heir apparent Though Otto I was crowned Emperor in 962 and returned to Germany in 965, the political situation in Italy remained unstable. After almost two years in Germany, Otto I made a third expedition to Italy in 966. Bruno was again appointed regent over the eleven-year-old Otto II during Otto I's absence. With his power over northern and central Italy secured, Otto I sought to clarify his relationship with the Byzantine Empire in the East. The Byzantine Emperor objected to Otto's use of the title "Emperor". The situation between East and West was finally resolved to share sovereignty over southern Italy. Otto I sought a marriage alliance between his Imperial house and the Eastern Macedonian dynasty. A prerequisite for the marriage alliance was the coronation of