The European Parliament opened in Strasbourg, France, this week to chaos. Outside, Catalan separatists protested the decision to bar their elected representatives from the chamber; inside, members of Britain’s Brexit Party turned their backs while the rest of the Parliament stood at attention for the union’s anthem, Beethoven’s “Ode to Joy.”

The disorder upstaged what was perhaps the most significant event of the day: the debut of a new alliance among Europe’s leading far-right nationalist groups. There, in the chamber, sat members of the populist far right, from Marine Le Pen’s National Rally, from France, to Matteo Salvini’s Northern League, from Italy. Their cooperation is worrying enough. But it also raises a question: Why are nationalists so eager to embrace an ethos of international cooperation?

For some in Europe, this alliance is mostly a pragmatic decision — undermining the European Union from within is not an easy task, and there’s strength in numbers. “We will not give up our identity; I think that unites us all,” said Jörg Meuthen, a member of the European Parliament from the Alternative for Germany party. The alliance’s collective aims: “no to further harmonization, no to the undermining of the nation state.”

But the cooperation goes beyond the specific goal of taking down Brussels, and beyond Europe. The group is supported by none other than Steve Bannon, the self-proclaimed éminence grise of the global far right. Nationalist leaders pop up at rallies around the world to support their local ideological cousins. Let’s remember the appearance of Mr. Salvini at a Trump rally in Philadelphia in 2016.