Civilization and commerce greatly suffered through the downfall of Rome, and did not again revive until after the struggles of the Northern Christian races with the Southern and Eastern nations, which had become Mohammedan. The sixth and seventh centuries were the darkest in the history of Europe. Charlemagne, toward the close of the eighth century, caused many of the old Roman roads to be repaired and new ones to be constructed. He, as well as several of his immediate successors, made use of mounted messengers to send imperial mandates from one part of the realm to the other. The rulers of the succeeding centuries did not profit, however, by this example, and the roads of the empire again fell into decay. Moreover, the public safety was greatly impaired by robbers and feudal knights, whose depredations were so heavy a tax upon commerce as to greatly discourage it. Trade under these circumstances would have been entirely destroyed, had it not been for the merchants' unions which were formed by the larger cities for the protection of their interests. These organizations maintained the most important thoroughfares, and even furnished armed escorts to wayfaring merchants. Commerce thus flourished in, and commercial relations were kept up among, the cities immediate between Venice and Genoa, as well as the cities on the Rhine and Danube. Florence, Verona, Milan, Strasbourg, Mayence, Augsburg, Ulm, Ratisbon, Vienna and Nuremberg were flourishing marts, and through them flowed the currents of trade between the north and the south. Out of these commercial unions grew in time the Hanseatic League, which from the thirteenth to the fifteenth century controlled the commerce of the northern part of Europe on both the water and the land. The object of this league, which at the height of its power included eighty-five cities, was to protect its members against the feudal lords on the land and against pirates on the sea. Its power extended from Norway to Belgium and from England to Russia. In all the principal towns on the highways of commerce the flag of the Hansa floated over its counting houses. Wherever its influence reached, its members controlled roads, mines, agriculture and manufactures. It often dictated terms to kings, and almost succeeded in monopolizing the trade of Europe north of Italy.