Complex sentence: Contented, the Byzantine emperor burned down the crops and returned to Constantinople but as soon he arrived there, Asen crossed the Danube with Cuman reinforcements and declared he would continue the struggle until all Bulgarian lands were liberated.
Split into simpler sentences: Contented, the Byzantine emperor burned the Bulgarians' crops and returned to Constantinople. Soon after, Asen crossed back over the Danube with Cuman reinforcements, declaring that he would continue the struggle until all Bulgarian lands were liberated.