But, apart from these favourable circumstances, the resources of Germany were irresistible when they could be concentrated. Twice after the Great Interregnum the integrity of the Empire was threatened by the Bohemian kingdom. On the first occasion, when Ottocar II had extended his power into the German lands between Bohemia and the Adriatic, he was overthrown by Rudolf of Hapsburg at the battle of the Marchfeld (1278); and a new Hapsburg principality was formed out of the reconquered lands to guard the south-east frontier against future incursions of Czech or Magyar. On the second, when the Hussite levies carried their devastations and their propaganda into all the neighbouring provinces of the Empire (1424-1434), crusade after crusade was launched against Bohemia until the heretics, uniformly victorious in the field, were worn out by the strain of their exertions against superior numbers, and all the more moderate spirits recognised that such triumphs must end in the ruin and depopulation of Bohemia. The case was the same in the Baltic, where the struggle with Danish ambitions was left to the princes and the free towns. Waldemar II (1202-1241), who had planned to revive the Scandinavian Empire of the great Canute, the conqueror of England, saw his ambitious edifice crumble to pieces while it was still in the making; even the Union of Kalmar (1397), by which the crowns of Norway and Sweden and Denmark were vested in a single dynasty, could not save the rich prize of the Baltic trade from falling into German hands. Germany, even when ill-governed and a prey to the ambitions of provincial dynasties, was still _grande chose et terrible_, as more than one political adventurer learned to his cost. The energy, the intelligence and the national spirit of a great people made good all the errors of statesmen and all the defects of institutions. [Illustration: Holy Roman Empire under Frederick Barbarossa]