The transition from medieval to modern history is most strongly marked by the emergence of this principle, with all that it involves. At the end of the Middle Ages Europe was at last compelled to admit that the grand idea of an universal state and an universal church had definitely broken down. Hitherto it had been assumed that behind all national disputes lay a _ius gentium_ by which all were bound, and that behind all religious questions lay the authority of the Roman Catholic Church, from which there was no appeal. The modern period which certainly does not represent the last word of civilisation, has witnessed the abandonment of these ideas. The change took place gradually. France became a nation when the English raids ceased in the middle of the 15th century. Spain achieved unity a generation later by the union of Castile and Aragon and the expulsion of the Moors from the peninsula. Holland found herself in the heroic struggle against Spain in the 16th century. But the practice of conducting wars by hiring foreign mercenaries, a sure sign that the nationalist spirit is weak, continued till much later. And the dynastic principle, which is the very negation of nationalism, actually culminated in the 18th century; and this is the true explanation of the feeble resistance which Europe offered to the French revolutionary armies, until Napoleon stirred up the dormant spirit of nationalism in the peoples whom he plundered. 'In the old European system,' says Lord Acton, 'the rights of nationalities were neither recognised by governments nor asserted by the people. The interests of the reigning families, not those of the nations, regulated the frontiers; and the administration was conducted generally without any reference to popular desires.' Marriage or conquest might unite the most diverse nations under one sovereign, such as Charles V.