This Empire had been a civilizing power. It had supported the Papacy, as an oak supports the creeper that clings to it; and in its decline and fall it pulled the Papacy down with it. Without such support the Papacy could maintain neither dignity abroad nor order at home. This lesson the Church learned once through the outrages inflicted upon Pope Leo, but forgot it; and required the experience of a hundred and fifty years to learn it a second time. In theory Papacy and Empire were co-equal powers, religious and secular, together carrying on the noble task of God's government on earth. In practice, as their respective rights and powers had not been definitely set off, they could not agree; each wished to be master. The relations between the two constitute the great axis on which mediaeval politics revolve, and for a long time must serve as the main motive of our story. The contest between them for mastery resembles a fencing match, in which the Pope thrusts at the Emperor's crown, the Emperor parries, and lunges back at the papal tiara. For convenience we divide the match into two bouts, and first take the Pope's attack. At the famous coronation on Christmas day, 800, Charlemagne and Leo stood side by side, co-labourers in the great task of reconstructing Europe. But once the coronation over, the two undefined authorities jostled each other. Charlemagne, to whom government was as much a religious as a secular matter, though he had accepted his Imperial crown at the hands of the Pope, did not regard papal participation necessary for the continuance of the Imperial dignity. At Aachen, 813, he crowned his son Louis the Pious co-Emperor, without the help of Pope or priest. This thrust must have carried discomfiture to the banks of the Tiber. But with Charlemagne's weak successors the astute Papacy scored hit after hit. Louis the Pious submitted to be recrowned by the Pope, so did his son, Lothair, and his grandson Louis II; and their two successors were also crowned by the Pope. This sequence of palpable hits won this bout and secured for the Papacy beyond dispute the prerogative of crowning the Emperors.