Right after the attacks in Brussels on Tuesday, Donald Trump did something bizarre. He spoke the truth. Appearing on Fox and Friends, the GOP presidential frontrunner declared that, “This all happened because frankly there is no assimilation.”

OK, Trump’s comment wasn’t entirely true. The attacks are also likely related to the fact that Belgium is participating in the U.S.-led bombing campaign in Iraq and Syria. ISIS said so when it claimed responsibility. And terrorism scholars have shown that the group doesn’t attack Western targets at random. It targets countries—in monstrous and utterly unjustifiable acts of murder—that are fighting it militarily in the Middle East.

Still, Trump’s point about assimilation was important. If many European countries struggle to integrate their Muslim immigrants, Belgium does a particularly poor job. The unemployment rate for Belgians of North and sub-Saharan African descent is between 40 and 50 percent. Last year, the BBC reported that of Antwerp’s 2,600 police officers, only 22 are non-white. In 2011, Belgium became the first country in Europe to ban the veil nationwide.

In an essay last year, the Nigerian-born writer Chika Unigwe, who lived for many years in Belgium, noted that even non-white people born there are often described as “foreigners.” She concluded that, “Assimilation, for a Belgian with non-European roots, is a near-impossible task.”

All this is terrific news for ISIS, which has called for eliminating the “grey zone” which allows Muslims to remain true to their faith while also living peacefully in the West.

So Trump’s diagnosis of the Brussels attack makes sense. Which is why the proposals that he and Ted Cruz offered in response to it are so idiotic. Soon after decrying Belgium’s lack of “assimilation,” Trump reiterated his call for temporarily banning Muslims from entering the United States. It’s hard to think of a proposal more likely to push America in Belgium’s direction. Today, American Muslims are far more integrated than Muslims in Europe. According to a 2011 Pew Research poll, only 20 percent of American Muslims surveyed would prefer to “be distinct” than to “adopt American customs.” Half say that many of their friends are non-Muslim. Almost 80 percent rate their community an “excellent” or “good” place to live.

Banning Muslim immigration would almost certainly undermine this. A 2014 study found that Muslim immigrants in states that experienced more anti-Muslim hate crimes were less likely to intermarry with non-Muslims and learn English. Trump’s demonization of Muslims has already fostered more of these anti-Muslim attacks, and were he to try to implement his ban on Muslim immigration, Islamophobia would likely spike even higher, undermining the very integration of American Muslims that helps keep America safe.

Ted Cruz’s proposed response to Brussels would have a similar effect. The day of the attacks, he called for police to “patrol and secure Muslim neighborhoods.” Asked what that meant, Cruz cited a program in New York that, according to The New York Times, allowed to the NYPD to designate “entire mosques as suspected ‘terrorism enterprises,’” and thus “collect the license plate numbers of every car in mosque parking lots, videotape worshipers coming and going, and record sermons using informants wearing hidden microphones.” What Cruz didn’t mention is that an NYPD official himself admitted the program didn’t yield a single terrorism investigation. What it did was alienate law-abiding Muslims. As a Newark-based FBI special agent noted, the program led “people [to] pull back cooperation” and thus impaired “our ability to have our finger on the pulse of what’s going on around the state.”

Compared to countries like Belgium, the degree of acceptance that American Muslims enjoy represents a form of American exceptionalism. Republican presidential candidates like to say Barack Obama doesn’t believe in American exceptionalism and instead wants to make America more like Europe. Yet it’s they who, by demonizing Muslims, would do just that.