Battle of Mohi (Fidem Pacis)

The Battle of Mohi was fought on the 11th April 1241 between the invading Mongol Empire and the forces of Roman Hungary. It is notable for being the first major battle in history in which gunpowder weaponry played an important role on both sides. It resulted in a narrow Roman victory, though the Mongols achieved their main goal of preventing Roman reinforcements from assisting Poland against a simultaneous invasion there.

The Mongols in Hungary returned to the steppes after the battle, though it's likely that they would have done so soon enough anyway after news of the death of Ogedei Khan arrived a few months later. Their worst casualty was the death of the great general Subutai, who had been the mastermind of many successful Mongol strategies up until then. Hungary meanwhile was left in ruins, accelerating its incorporation into the Roman Empire proper.