Young women hold each other at a makeshift memorial in front of the stock exchange at the Place de la Bourse (Beursplein) in Brussels on March 22, 2016, following triple bomb attacks in the Belgian capital that killed about 35 people and left more than 200 people wounded. (Kenzo Tribouillard/AFP/Getty Images)

THE TERRORIST assault on Brussels Tuesday, just four days after the arrest of an architect of last year’s attacks in Paris, underlined the resilience and continued menace of the Islamic State — to Europe, to the United States and to vital Western interests. It also revealed a crucial divide among U.S. presidential candidates about what this country must do to protect itself.

One one side are those who support the internationalist response of President Obama, who said the United States “will do whatever is necessary to support our friend and ally Belgium,” and who asserted that “we must be together, regardless of nationality or race or faith, in fighting against the scourge of terrorism.” That view was broadly shared by Democrats Hillary Clinton and Bernie Sanders and Republican John Kasich.

Against them is the radical isolationism of Donald Trump, from whom the Brussels bloodshed prompted another call to “close up our borders,” and who on Monday questioned the value of U.S. support for NATO allies such as Belgium. Though GOP rival Ted Cruz rejected Mr. Trump’s position on NATO, his answer to Brussels was similar: He, too, stressed “secur[ing] the southern border” and curtailing refu­gee flows, along with patrols of “Muslim neighborhoods.”

More than at any time since 1940, America’s commitment to its European allies is at issue in a presidential campaign. The tragic events of Brussels illuminate the folly of Mr. Trump’s position. The Islamic State has targeted all Western democracies, along with Israel and the Sunni states of the Middle East; it regards Belgians and Americans equally as enemies. Destroying the group — as Mr. Trump says is necessary — cannot be done without fighting its tendrils wherever they appear — in Europe as well as the Middle East, in Africa and in cyberspace. However much they are reinforced, borders will provide no protection to Americans if the jihadists are not defeated elsewhere.

Mr. Trump protests that NATO “is costing us a fortune” and that the United States is no longer a rich country. Never mind that the nation is far richer than it was when the alliance was set up in 1949, or that the national debt as well as spending on defense are lower as a portion of the economy. To defeat the Islamic State without NATO’s help would impose huge costs on Americans. Britain, France and Germany, among others, contribute materially to the war against the terrorist entity in Iraq and Syria, not to mention NATO member Turkey.

Listen to Republican presidential candidate Donald Trump discuss some of his foreign policy positions with The Washington Post editorial board. "NATO is costing us a fortune," Trump said. "We're not reimbursed fairly for what we do." (The Washington Post)

Intelligence sharing among the allies is critical to disrupting plots in the United States as well as elsewhere. Mr. Trump told us he saw no advantage to U.S. foreign bases; yet without those provided by Turkey, the air campaign in Iraq and Syria would be far less effective.

Mr. Trump is not the only one to complain about the way the defense burden is divided among NATO’s members. President Obama has also griped about “free riders.” The next U.S. president must keep pressing allies to spend more on defense and to commit to operations against Islamic State bases in places such as Libya. But she or he must also accept that the alliance won’t function without U.S. leadership — which inevitably means a larger role militarily and financially as well as politically.

“Why are we always the one that’s leading?” complained Mr. Trump. The simple answer is that, in the absence of that American commitment, chaos like that seen in Brussels will soon cross even our most fortified borders.