Stirrups
In 1066 one of the most decisive battles in the history of the world was fought. William, Duke or Normandy, ventured an invasion of England in the face of a formidable opponent. But one of the reasons that gave him the confidence to try such a risky undertaking was that he had a recently invented technological edge that the English did not. That edge was the stirrup.
While the English rode to the battlefield, they fought on foot; conventional wisdom being that the horse was too unstable a platform from which to fight. But the Norman cavalry, standing secure in their stirrups, were able to ride down the English, letting the weight of their charging horses punch their lances home. This technological edge led to the conquest of Britain. Without it, William might never have attempted such a perilous war.