Complex sentence: He defeated the army of Flanders that had invaded Walcheren in 1253, where he captured Guy and John of Dampierre, both sons of Margaret II, Countess of Flanders and commanders for Flanders.
Split into simpler sentences: He defeated the army of Flanders that had invaded Walcheren in 1253. He captured Gwijde and Jan of Dampierre, both sons of Margaret II, Countess of Flanders and commanders for Flanders.