LONDON—I was in London on Sept. 11, 2001. It was impossible to call home because the lines were down; in that pre-smartphone era, it was also impossible to know what was going on, unless there was a television screen in the vicinity. Cut off though I was, I felt surrounded by friends. Upon hearing my accent, shop assistants and taxi drivers asked after my parents: Had I spoken to them yet? Could they help? That night, the Tory party called off its leadership election; the German chancellor spoke of a “war against the entire civilized world.” The NATO ambassadors, meeting in Brussels, unanimously invoked the NATO treaty: An attack on one member state is an attack on all.

On March 22, 2016, I was in London once again, watching another generation of Islamic terrorists carry out another series of coordinated attacks in Brussels, the capital of Belgium and the European Union as well as the headquarters of NATO. This time it was impossible not to follow the news and not to see photographs of everything that happened, almost in real time. It was also impossible not to notice the reactions, which came a lot faster than they used to do. Some were reminiscent of 2001. A Belgian flag was raised over 10 Downing Street, the residence of the British prime minister; the presidents of the U.S. and France spoke almost immediately in support of the Belgian government, too.

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 U.K. Independence Party declared that the open borders of Europe “are a threat to our security,” even though the U.K. 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 EU on the grounds of safety. Meanwhile, American news organizations fell over themselves to get instant reactions from Donald Trump, who had just told the Washington Post that he didn’t see the point of NATO, which “is costing us a fortune.” He didn’t disappoint: “[W]e 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 is presented 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 European Union institutions. Never mind that the U.S. 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, very few security threats can be stopped by border guards anyway. Every terrorist attack on U.K. 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, short-sighted isolationists ignore reason and logic, instead substituting panic and fear.

Of course there are reasons for this change: German Chancellor Angela Merkel’s disastrous decision to apparently “invite” Syrian immigrants into Europe last summer has left many Europeans feeling queasy and out of control. Photographs from the war in Syria and the refugee camps in Greece have upset even people living in countries such as the United States that have not accepted large numbers of refugees. But those are explanations, not an excuse, for the stupidity of isolationism. We don’t have a choice: The only way to fight jihadism is through our existing military, economic, and political alliances. And the only way to ensure that we have international support in the future, when a tragedy takes place on our soil—and it will—is to offer our support for a tragedy unfolding on allied soil right now.

Read more of Slate’s coverage of the Brussels terror attacks.