732 AD
In 732 AD, the Umayyad Caliphate's forces, having conquered the Iberian Peninsula (Spain and Portugal) and pushed into southern France, were decisively defeated by the Frankish army led by Charles Martel ('the Hammer') at the Battle of Tours (also called the Battle of Poitiers). The Muslim commander, Abd al-Rahman al-Ghafiqi, was killed, and the Umayyad forces retreated south of the Pyrenees.
This battle is often cited as the turning point that saved Western Europe from Islamic conquest and preserved Christianity as the dominant faith of the continent. While modern historians debate how far the Umayyads would have penetrated even without this defeat, the battle certainly consolidated Frankish power under the Carolingians, whose dynasty would produce Charlemagne.
Charles Martel's victory ensured that the Merovingian Frankish kingdom would evolve into the Carolingian Empire, which would become the foundation of medieval Christendom. Key Sources: The Chronicle of Moissac; Fredegar Chronicle; Einhard, Life of Charlemagne