The coalition between the Empire and the Church, which, after various tentative preliminaries, was finally effected by Constantine, launched the world upon new paths: and his transference of the main seat of empire to the shores of the Bosporus left Western Europe to pursue fragmentary and independent courses. The Latin-speaking provinces were falling away in great lumps. An independent empire of Britain had already existed for six or seven years under the usurper Carausius. After the middle of the fourth century Gaul was practically in possession of the Visigoths and the Salian Franks. During the reign of Honorius mixed hordes of Vandals, Suabians, and Alans poured through Gaul across the Pyrenees, and divided Spain into barbarian monarchies. A few years later the Vandals, called across the Straits of Gibraltar by the treachery of Count Boniface, overran the province of Africa, and established a powerful kingdom, whose fleets, issuing from the port of Carthage, swept the Mediterranean and sacked Rome itself. Rome had, by the famous edict of Antoninus Caracalla, given the world a single citizenship; to give organic life to that citizenship, and turn her citizens into a single nation, was a task beyond her power. So long as the Latin-speaking world remained nominally subject to a single rule, exercised in the name of the Senate and People of Rome, Latin literature had some slight external bond of unity; after the Western Empire was shattered into a dozen independent kingdoms, the phrase almost ceases to have any real meaning. Latin, in one form or another, remained an almost universal language; but we must speak henceforth of the literatures of France or Spain or Britain, whether the work produced be written in a provincial dialect or in the international language handed down from the Empire and preserved by the Church. For the Catholic Church now became the centre of European cohesion, and gave continuity and common life to the scattered remains of the ancient civilisation. Already, in the fifth century, Pope Leo the Great is a more important figure than his contemporary, Valentinian the Second, for thirty years the shadowy and impotent Emperor of the West. Christian literature had taken firm root while the classical tradition was still strong; in the hands of men like Jerome and Augustine that tradition was caught up from the wreck of the Empire and handed down, not unimpaired, yet still in prodigious force and vitality, to the modern world.