The idea of a united Europe, embodied in the European Union, is noble. The idea of a borderless Europe is dangerous.
A classic case of the law of unintended consequences: Blur the borders of Europe in order to eliminate the relentless wars among its nation states, and end up with terrorists easily slipping across those borders with their lethal weapons.
From Caesar to Charlemagne to the founders of the League of Nations, the idea was to eliminate the causes of frequent, bloody national conflict on the continent. Beginning with the visionary Jean Monet and his post-WW II Coal and Steel Community, the European Union gradually came into being. How wonderful, how naive, it sounded: Let the Germans share their coking coal with the French, and let the French share their iron ore with Germany, and both nations (and many other participants in the “community”) would make steel. And while they were making steel, they would not make war.