Charlemagne, having disposed of the Lombards, continued his conquests; across the Pyrenees he annexed the Spanish March, in North Germany he subdued the Saxons and pushed his frontier to the Elbe, to the southeast he subjugated the country as far as the upper Danube. His monarchy now included Franks, Celts, Visigoths, Burgundians, Saxons, Lombards, Romans. How were such widespread territories and such diverse peoples to be united in permanent union? The far-seeing Papacy, in answer to this question, propounded the revival of the Roman Empire of the Caesars. Reasons were numerous. The Frankish monarchy, with its conquests, in bulk at least was not unworthy to succeed to Imperial Rome. Throughout this wide territory there was a great network of ligaments; from Gascony to Bavaria, from Lombardy to Frisia, divine service was celebrated in the Latin tongue and with the Roman ritual; bishops, priests, monks, and missionaries acknowledged their dependence upon the Pope and looked to Rome, with its holy basilicas and apostolic tradition, as the centre of Christendom. This Christian unity was a constant argument for political unity. A second argument was the still vigorous Roman tradition. The idea of nationality was as yet undeveloped; Europe had known no other political system than common subjection to the Roman Empire, and all notions of civilization were of a civilization on the Roman pattern. When the Roman Empire in the West had decayed, the Church had adopted the Imperial organization and kept remembrance of the old system fresh in men's minds. The old Empire, moreover, had early lost the notion of dependence on the city of Rome, for the seat of government had been set at Constantinople, at Milan, and at Ravenna; and since the days of the early Caesars, it had not been necessary for an Emperor to be a native Roman. There was no theoretical difficulty to bar a Frank from the Imperial throne or forbid the seat of government to a Frankish city. In fact, nobody could conceive of the Empire as other than Roman, and the Frankish kingdom could only become an empire by becoming the Roman Empire.