A new force that redrew the map of the continent.
For most of European history, ordinary people did not think of themselves as members of a nation. A Bavarian peasant in 1750 owed allegiance to his local lord, his parish priest, and his prince. He spoke a dialect that might be unintelligible fifty kilometers away. He had never seen a map of "Germany" because no such country existed. If you had told him he shared a national destiny with a Prussian merchant, a Swabian blacksmith, and a Saxon miner, he would have looked at you with genuine confusion.
Political loyalty ran upward through personal bonds: serf to lord, lord to king, king to God. States were dynastic possessions, not expressions of popular identity. The Habsburg family ruled a patchwork of territories stretching from the Netherlands to Hungary, encompassing Germans, Czechs, Hungarians, Poles, Italians, Croats, Slovaks, and Romanians. Nobody expected these groups to share a single political identity. Nobody thought they should.
The French Revolution changed that calculation permanently.