Louisiana has a striking reminder of the blood and treasure America has spent to advance a peaceful and prosperous Europe. It’s the National World War II Museum in New Orleans, which honors the warriors who did so much to nurture a Europe built on common ideals, not conflict and distrust.

That goal continues to be important for America and Louisiana, which is why the results of Great Britain’s Thursday vote on its future with the European Union are so unsettling. In postwar Europe, the twin pillars of unity have been NATO, its shared defense pact, and the EU, an agreement among the region’s major players to advance free trade.

In deciding to exit the European Union, Britain’s voters have called the future of that alliance into question. It’s a development with economic implications for the United States, and — perhaps down the line — impacts on global security, too.

In a national referendum, voters in Britain turned thumbs-down on the EU, unhappy with its bloated bureaucracy and sometimes onerous rules. Anti-immigrant feeling figured into the vote. Many British advocates for leaving the EU argued that an exit from the Union would give Britain more direct control of its borders.

As anyone who’s followed this year’s American presidential campaign already knows, walls, not windows, are the political fashion of the day. It’s the kind of thinking that sees the world as a constellation of fortresses, not a common market of goods and ideas.

That’s not good for global commerce, an arena in which Louisiana has an obvious stake.

In the wake of the Brexit vote, America will continue to have an important interest in cultivating close ties between Britain and its European allies. It’s what we owe the members of the Greatest Generation who gave their lives to build opportunities for the future, not obstacles.