William I’s foreign policy

Concerning foreign policy, William pursued the same Mediterranean and Eastern policy as his father. The Byzantines tried to take advantage of the problems of his reign by conquering Apulia. This led to a victorious counter-offensive by William in 1156, near Brindisi. He continued his offensive, and in 1156, the Catepan of Apulia, the brother of Maio of Bari (the king’s principal counsellor) waged war in the Aegean Sea with an army of 10,000 men to seek revenge on the basileus. In 1158, the latter was obliged to sign a peace treaty of thirty years and to recognise the rights of the Hautevilles over the former Greek possessions in Italy. 
This recognition by Byzantium was accompanied by that of the papacy. Isolated and confronted with manoeuvres from the Germans after the defeat in Brindisi, Pope Adrian IV needed powerful protectors. He was obliged to come to terms with William: in the treaty of Benevento (June 1156) with the investiture of the kingdom, including Capua and Naples and also confirmed the special status of Sicily, held by delegation from the Holy See since the conquest of Roger the Grand Count.
The situation on the mainland and Sicily was finally stabilised, but the victorious advances of the Almohades, who wanted to unify North Africa and conquer all the Norman forts, were challenging the Norman tutelage of the possessions on the southern shores of the Mediterranean. The last one, Mahdia, fell in January 1160. This was the end of the Norman Mediterranean region.