[2] In 1218, when his older half-brother Valdemar the Young was crowned king as their father's co-ruler and designated heir, he was created Duke of Schleswig.

After the premature death of Valdemar in 1231, Eric was crowned king at Lund Cathedral 30 May 1232 as his father's co-ruler and heir.

[3] His rule was marked by bitter conflicts, especially against his brother, Duke Abel of Schleswig who seems to have wanted an independent position and who was supported by the counts of Holstein.

The conflict lasted for three years before the brothers agreed on a truce in 1244 and made plans for a joint crusade to Estonia.

[5] Infuriated, in 1249 King Eric directed his rage at Niels Stigsen, Bishop of the Diocese of Roskilde who fled Denmark the same year.

In spite of intervention from Pope Innocent IV who advocated the reinstatement of the bishop and the return of the properties to the diocese, the dispute could not be resolved.

The conflict started when Eric invaded Holstein in an attempt to restore his father's control of the county.

The following year, Abel and the Holsteiners stormed into Jutland and Funen, burning and pillaging as far north as Randers and Odense.

Abel was supported by the Hanseatic League city of Lübeck, as well as by his brothers Christopher, Lord of Lolland and Falster and Canute, Duke of Blekinge.

A truce was arranged by Eric's sister Sophie of Denmark (ca 1217–1247) who was the wife of Johann I, Margrave of Brandenburg (c. 1213–1266).

On his way home in 1250 he took his army to Holstein to prevent the capture of the border fortress of Rendsburg and to teach the German counts who was still king.