This left Henry III as his father's only surviving son, so he declared himself the heir, not only of Holstein-Rendsburg, but also of the Duchy of Schleswig, which had been given as a hereditary fief to Gerhard VI in 1386.

On 12 August 1410, armies from Holstein-Rendsburg and Denmark fought a battle in moorland near Sollerup in the district of Eggebek.

Holstein-Rendsburg won, and the Danish army leader Mogens Munk fell in battle.

Margaret I died in October 1412 and the nobility from Holstein refused to return the territories they'd occupied in 1410, as had been agreed in the compromise of 1411.

Eric I took the case to a feudal court in Nyborg, which ruled on 29 July 1413 that Henry III had forfeited his duchy.