Two scenes, seven centuries apart, fix excommunication's political history. In 390, after imperial troops massacred thousands in the circus at Thessalonica, Ambrose of Milan barred the emperor Theodosius from communion until he did public penance, and the emperor submitted, laying aside the purple in the cathedral. The scene established the claim from which everything later flowed: no one, however sovereign, stands above the discipline of the church. In the winter of 1077 the claim reached its theatrical summit at Canossa, where Henry IV, excommunicated and deposed by Gregory VII in their war over lay investiture, stood as a barefoot penitent in the snow of the castle courtyard, by the accounts for three days, until the pope absolved him.
Canossa became a proverb in several languages precisely because both sides could read it as victory: the papacy had brought an emperor to his knees; the emperor, absolved, kept his crown and renewed the war. The instrument's political potency depended entirely on shared belief, an excommunicated ruler was dangerous to follow, since oaths to him might be loosed, and it decayed as that belief did. By 1570, when Pius V excommunicated Elizabeth I and purported to release her subjects, the bull mostly worsened the position of English Catholics; the weapon fired backward. Modern excommunication has retreated from thrones to its older ground, the discipline of communion, where the scene at the church door began.