It was, this time, no central European power, to which, at the worst, all that lay north of the proper Byzantine sphere might be abandoned; but a claimant for part of that sphere itself, perhaps even for the very heart of it. Russia, seeking an economic outlet, had sapped her way south to the Euxine shore, and was on the point of challenging the Osmanli right to that sea. The contest would involve a vital issue; and if the Porte did not yet grasp this fact, others had grasped it. The famous ‘Testament of Peter the Great’ may or may not be a genuine document; but, in either case, it proves that certain views about the necessary policy of Russia in the Byzantine area, which became commonplaces of western political thinkers as the eighteenth century advanced, were already familiar to east European minds in the earlier part of that century. Battle was not long in being joined. In the event, it would cost Russia about sixty years of strenuous effort to reduce the Byzantine power of the Osmanlis to a condition little better than that in which Osman had found the Byzantine power of the Greeks four centuries before. During the first two-thirds of this period the contest was waged not unequally. By the Treaty of Belgrade, in 1739, Sultan Mahmud I appeared for a moment even to have gained the whole issue, Russia agreeing to her own exclusion from the Black Sea, and from interference in the Danubian principalities. But the success could not be sustained. Repeated effort was rapidly exhausting Osmanli strength, sapped as it was by increasing internal disease: and when a crisis arrived with the accession of the Empress Catherine, it proved too weak to meet it. During the ten years following 1764 Osmanli hold on the Black Sea was lost irretrievably. After the destruction of the fleet at Chesme the Crimea became untenable and was abandoned to the brief mercies of Russia: and with a veiled Russian protectorate established in the Danubian principalities, and an open Russian occupation in Morean ports, Constantinople had lost once more her own seas. When Selim III was set on a tottering throne, in 1787, the wheel of Byzantine destiny seemed to have come again almost full circle: and the world was expecting a Muscovite succession to that empire which had acknowledged already the Roman, the Greek, and the Osmanli.