Complex sentence: In 1225, Honorius III called the Hungarians to undertake the Bosnian Crusade that failed when the Hungarians retreated following defeat by the Mongols at the Battle of Mohi.
Split into simpler sentences: In 1225, Honorius III called the Hungarians to undertake the Bosnian Crusade. That expedition turned into a defeat when the Hungarians had to retreat following defeat by the Mongols at the Battle of Mohi.