He was the son of Baldwin III of Flanders and Matilda of Saxony. Baldwin III died in 962, when Arnulf was just an infant, and with Arnulf's grandfather count Arnulf I of Flanders still alive. When Arnulf I died two years later (964), the regency was held by their kinsman Baldwin Balso.
By the time Arnulf attained his majority in 976, Flanders had lost some of the southern territory acquired by Arnulf I. The latter had given some parts of Picardy to King Lothar of France to help assure his grandson's succession, and gave Boulogne as a fief to another relative. Then early in Arnulf's minority Lothar had taken Ponthieu and given it to Hugh Capet, and the first counts of Guines had established themselves.
He married Rozala of Lombardy, daughter of Berengar II of Italy, and was succeeded by their son, Baldwin IV.
Margaret II marries the Emperor Louis IV, of the Bavarian Wittelsbachs.
Albert II The modern identity of Belgium was forged in the revolt of the Netherlands against Spain.
A German prince, Leopold of Saxe-Coburg, an uncle of Queen Victoria of England, married to a daughter of the new Bourgeois King of France, was brought in to create a monarchy, and, indeed, Belgium grew in its neutrality for the rest of the century.