Complex sentence: Nevertheless, he sent 4,000 horsemen under the command of his son, Mircea, to fight against the Ottomans in the Battle of Varna, which ended with the catastrophic defeat of the crusaders on 10 November 1444.
Split into simpler sentences: Nevertheless, he sent 4,000 horsemen under the command of his son, Mircea, to fight against the Ottomans. The crusade ended with the catastrophic defeat of the crusaders in the Battle of Varna on 10 November 1444.