We knew within hours of the attack that the New York truck killer was a Muslim extremist, inspired by ISIS. In today’s climate, that means that NPR was hesitant to report that he had shouted “Allahu akbar” and that Donald Trump thundered that our legal system is a “laughingstock” and blamed immigration.

The left fears that any terror attack will be exploited to stoke animosity toward Muslims and immigrants. President Trump obliges by vowing to end the diversity lottery and fulminating about the death penalty (thus making it more difficult for prosecutors to secure capital punishment, but oh well). Trump embodies the caricature of the ignorant bigot. It’s a stark contrast to the prudent response of President George W. Bush to 9/11. Bush was resolute against our enemies yet determined not to scapegoat our friends.

That said, the left suffers from a crippling blindness about the link between ideology and violence. Oh, they can see it well enough when the ideology is white supremacy. And in their fevered imaginations, they saw even mild tea party protesters as a dire threat. But what they cannot process is that ideas rule the world, and given the powerful human inclination toward cruelty, ideologies that justify violence are deeply malignant.

The New York attacker apparently entered this country in 2010 and over the past several years became radicalized by ISIS propaganda he watched online. His phone reportedly contained 90 videos, including instructions for bomb making and pictures of Abu Bakr al-Baghdadi, the leader of ISIS. If this man had been a recent convert to Buddhism or Christianity, his religious convictions would have been a matter of zero interest to anyone but him, because those religions do not justify mass killing. Yes, Christianity once did justify holy war. And when it did, it was dangerous. But it hasn’t been the regnant ideology of Christianity for more than 700 years. That doesn’t mean some Christians don’t still commit crimes in the name of faith, but those who do are extremely rare.

People love violence but also want to believe in their own virtue. That’s why ends-justify-the-means ideologies are so dangerous. The communists taught that they were inaugurating a new and more benevolent world, and if some kulaks and bourgeois capitalists were liquidated in the process, it was all in a good cause.

So, too, with Islamism. It differs from the religion of Islam in its conspiracy-mongering, its belief that sharia law should prevail and its thirst for martyrdom. It promises the monsters among us that they are doing nothing less than God’s will by acting out their most vicious fantasies. If all of the world’s more than 1 billion Muslims were Islamists, there would be no peace anywhere on the planet. In fact, according to estimates by Daniel Pipes of the Middle East Forum, only about 10 to 15 percent of Muslims worldwide are radicals.

That still leaves room for 100 million Islamists or people who tolerate Islamism. It is not bigotry to notice that the virus of extreme religious intolerance is virulent in Islam today.

Careful vetting of immigrants makes sense, while a meat-ax approach does not. This struggle may continue for decades or even centuries. Unlike in the contest with communism, we in the non-Muslim world have limited influence. Our best hopes for countering Islamism are: 1) recognizing that it’s an ideology; and 2) remembering that moderate Muslims are indispensable allies.