"U.S. Homeland Security officials are working with groups around the United States to head off any possible anti-Muslim backlash following the shootings at Fort Hood in Texas."

The Department of Homeland Security is in good company in its confusion. Gen. George Casey, the Army's top general, also worried that "this increased speculation could cause a backlash against some of our Muslim soldiers. And I've asked our Army leaders to be on the lookout for that." And President Obama cautioned against "jumping to conclusions."

The backlash trope is trotted out after every episode of terrorist violence. But it is as false as it is dangerous. This image of a nation on a hair trigger for violence against Muslims is a calumny. Even in the immediate aftermath of 9/11, though millions were inflamed by grief and outrage, there was no broad-based "backlash" against Muslim Americans. There were a handful of crimes including the murder of a Sikh who may have been mistaken for a Muslim, a few broken windows, some insults, and some hurt feelings. But the overwhelming majority of Americans did not seek out scapegoats, nor engage in vigilantism.

The repeated invocation of this libel has had an effect, though. It has succeeded in intimidating many Americans about the proper bounds of discussion. Gen. Casey reinforces this timidity when he frets that "our diversity" may be a casualty of the attack at Fort Hood. He and the Obama administration are obscuring the real challenge Americans face.

Our challenge is not to transcend the demons of vengeance clawing at our souls. Our challenge is to deal intelligently with a threat that arises from religious convictions. Non-bigoted observers can see that while the vast majority of the world's Muslims are not extremists, a significant minority are. And it matters what people believe.

We don't like to pass judgment on others' religious convictions. That's fine. But when a religious belief spurs violence and mass murder, it becomes political, and it becomes a proper concern of the military and security services.

Worldwide, Muslims believing themselves to be advancing the faith have committed more than 14,000 acts of violence just since 9/11. You know the litany: Madrid, London, Bali, Jerusalem, Mumbai, Amman. The list is long and bloody -- and it includes many innocent Muslims.

Many hit home. In 2003, Hasan Akbar, a Muslim convert, rolled a grenade into the tent of his fellow soldiers in the 101st Airborne Division on the eve of the invasion of Iraq. In June, Abdulhakim Muhammad, another convert, killed one Army recruiter and wounded another in Little Rock. Naveed Haq shot six women at the Seattle Jewish Federation office in 2006.

Federal agents have thwarted planned terror attacks on Fort Dix, N.J., folded up a terror ring in Lackawanna, N.Y., and uncovered plots against the nation's financial centers, the World Bank, the Sears Tower, the New York subway system, the Los Angeles airport, the Israeli Consulate in Los Angeles, 10 airliners landing in the U.S. (the liquid bomb plot), JFK airport, the Brooklyn Bridge, and the Prudential Building in Newark, N.J., among others.

So shall we arrest all the Muslims in America? That's the caricature that is encouraged by the "backlash" peddlers. Obviously not. But what we must do is to discriminate -- that is, to make distinctions based on what kind of Islam Muslims embrace. We have created a climate in which members of the military were afraid to raise questions about the bald and blatant Islamist comments Major Nidal Hasan expressed over many years. He was overhead saying, "maybe people should strap bombs on themselves and go to Times Square." He was caught proselytizing his patients. He argued frequently to colleagues that the U.S. was engaged in a "war against Islam."

Yet no one raised a red flag. Might be interpreted as anti-Muslim bigotry. And so the military took no action against a man who loudly advertised his extremist sympathies. Thirteen Americans paid for that with their lives. If any good were to come out of the Fort Hood massacre, it would be a new clarity about what we are fighting. Islamism is the enemy. Moderate Muslims are allies in the cause. We should no more shrink from confronting and battling Islamism than we would from any of the "isms" we destroyed in the 20th century. Muddled thinking and misplaced delicacy have proved deadly.