This was the crowning achievement of his last years of work. The problem of the end of Antiquity and the beginning of the Middle Ages had always preoccupied him. Before the War, in his lectures on the History of the Middle Ages, he drew attention to the profound traces which the institutions of the late Roman Empire had left upon those of the Frankish epoch. But is was during his captivity in Germany, when, as a prisoner in the camp of Holzminden, he organized, for the many Russian students who shared his fate, a course of lectures on the economic history of Europe, that the solution of this capital problem seems to have dawned upon him. And during his exile in the village of Kreuzburg, in Thuringia, while he was writing his History of Europe, he emphasized, for the first time, the close relation that existed between the conquests of Islam and the formation of the mediaeval Occident.

The fragility of papyrus in the northern climate explains why so little is left; but we must be under no illusion as to the quantity which was formerly employed. And the wealth of information which we possess concerning Gaul, thanks to Gregory of Tours, should not make us forget the fact that still more papyrus must have been used in Italy and Spain, and that this must have necessitated an exceptionally active import trade.

Another article of consumption figured very largely in the commerce of the period. This was oil. There was a demand for it everywhere; in the first place for alimentary purposes, for it seems that in Southern Gaul nearly everything was cooked in oil, as in Spain and Italy. The native olive trees did not suffice to meet the demand. It was necessary to import oil from abroad; all the more necessary inasmuch as the lighting of the churches at this period - doubtless because oil was so abundant - was not by means of wax candles, as it was at a later date, but by means of oil-fed lamps. Now Africa was the greatest oil-producing country in the Empire, and was to remain so until the Musulman conquest. It was exported from Africa in orcae. Theodoric, between 509 and 511, wrote to the Bishop of Salona on behalf of the merchant Johannes, who had furnished this bishop with sexaginta orcas olei ad implenda luminaria, and who wanted to be paid. The rest of the letter shows this was merely a parvitas, that is to say, a bagatelle. Gregory of Tours tells us something about the oil trade at Marseilles; he speaks of a merchant who had seventy orcae of oil stolen on the quay. A diploma grant by Clovis III, in 692, and renewed in 716, but which really dated back to Dagobert I (d. 639) granted the monastery of Saint-Denis an annual subsidy of a hundred solidi, with which the actors regii were to buy oil from the cellarium fisci, in accordance with the ordo cataboli. A formula of Marculf's mentions Marseilles as the port where the oil for the luminaria was generally purchased.

This oil therefore found its way even into the North. The Corbie Diploma of 716, which mentions 10,000 pounds of oil, is further proof of this. It must not be supposed that this was oil from Provence, for it was deposited in the cellarium fisci. A tax which refers to the exportation of oil from Bordeaux gives us reason to believe that this oil was forwarded from Marseilles.

The year 751 saw the alliance of the Carolingians with the Papacy. It was planned under Zacharias and completed under Stephen II. Before the situation could be completely reversed the last thread that connected the Pope with the Empire had to be broken, for so long as it existed the Papacy was force to remain, in defiance of its nature a Mediterranean power. It doubtless would have remained a Mediterranean power if Islam had not robbed it of Africa and Spain. But Germany, in the North, was now of greater significance.

Still, tradition was so powerful that if by some impossibility the Emperor could have driven out the Lombards, the Pope would have remained faithful to him. But in 749, with the appearance of Aistulf, the Lombards resumed their policy of conquest.

In 751 they seized Ravenna, and this time they remained permanently in possession of the city. Rome could no longer evade her fate. In 752 Aistulf's army was before the walls. Only immediate assistance could save the city. Stephen began by imploring help of the Iconoclast. He begged him to come with an army and save the city of Rome. But Constantine V contented himself with sending an embassy to the Lombards. Aistulf received it, but refused to make any concession. Pope Stephen II then appealed to Pippin for assistance, but before taking this decisive step he himself went to Pavia, where he besought Aistulf to renounce his conquests. Having suffered a rebuff, he left for the court of Pippin, where he arrived in January 754.

The inevitable had come to pass at last. The tradition which Pippin had broken in 751 was abandoned three years later by the Pope himself.