But among those first responses there was also a new tone, one that was definitely missing in 2001, and one that wasn’t even noticeable after the Paris attacks a few months ago. Instead of calling for solidarity against a common threat, a spokesman for the anti-European UK Independence Party declared that the open borders of Europe “are a threat to our security,” even though Britain is not part of Europe’s Schengen border treaty. A columnist for the Daily Telegraph declared Brussels the “jihadist capital of Europe” and mocked those who call for staying in the E.U. on the grounds of safety. Meanwhile, U.S. news organizations fell over themselves to get instant reactions from Donald Trump, who had just told The Post that he didn’t see the point of NATO, which “is costing us a fortune.” He didn’t disappoint, declaring that “we have to be very careful and very vigilant as to who we allow in this country.”

On both sides of the Atlantic, isolationism is now a fact of political life. Although it gets expressed differently in different places, the illogical idea that “my country will be safer” if it pulls out of its international alliances is growing. Never mind that Britain constantly shares intelligence and information on terrorism with the rest of Europe via E.U. institutions. Never mind that the United States works with NATO allies to track terrorist operations and deter attacks, or that we gain enormous security as well as economic benefits for doing so. Never mind that, nowadays, few security threats can be stopped by border guards anyway. Every terrorist attack on British soil in recent memory was carried out by British (or Irish) citizens and not foreigners; nuclear deterrence requires allies and coordinated responses; barbed wire cannot stop a cyberattack. The small-minded, shortsighted isolationists ignore reason and logic, instead substituting panic and fear.