The day after last week’s attack in Boston, David Sirota wrote a column for Salon entitled “Let’s Hope the Boston Marathon Bomber Is a White American,” arguing that this would limit the resulting crackdown on civil liberties. At first, conservatives were appalled. Then, when police fingered the Tsarnaev brothers, they were triumphant. “Sorry, David Sirota, Looks Like Boston Bombing Suspects Not White Americans,” snickered a headline in Newsbusters. “Despite the most fervent hopes of some writers over at Salon.com,” added a blogger at Commentary, “the perpetrators of the Boston Marathon bombing are not ‘white Americans’.”

But the bombers were white Americans. The Tsarnaev brothers had lived in the United States for more than a decade. Dzhokhar was a U.S. citizen. Tamerlan was a legal permanent resident in the process of applying for citizenship. And as countless commentators have noted, the Tsarnaevs hail from the Caucasus, and are therefore, literally, “Caucasian.” You can’t get whiter than that.

So why did conservatives mock Sirota for being wrong? Because in public conversation in America today, “Islam” is a racial term. Being Muslim doesn’t just mean not being Christian or Jewish. It means not being white.

Think about American history and you can understand why. For centuries, Americans were legally segregated by race. Thus, when newcomers from the Middle East came to our shores, Americans had to decide which side of the line they were on. And in the struggle to be classified as white, Middle Eastern Christians had an advantage: Jesus. In the 1915 case Dow v. United States, a Syrian Christian successfully argued that he was white because Jesus, the original Middle Eastern Christian, was too. In 1925, in United States v. Cartozian, the court designated Armenians as white because, “[a]lthough the Armenian province is within the confines of the Turkish Empire, being in Asia Minor, the people thereof have always held themselves aloof from Turks, the Kurds, and allied peoples, principally, it might be said, on account of their [Christian] religion.” In the 1942 case In Re Ahmed Hassan, a Michigan court said a petitioner from Yemen was not white because “Apart from the dark skin of the Arabs, it is well known that they are a part of the Mohammedan world and that a wide gulf separates their culture from that of the predominately Christian peoples of Europe.”

Today, Americans still often link Islam and dark skin. What’s changed is which category we consider more dangerous. For much of American history, the problem with being Muslim was that you weren’t considered white. Since 9/11, by contrast, one of the problems with not being considered white is that you might be mistaken for Muslim. Thus, four days after the Twin Towers fell, Balbir Singh Sodhi, a Sikh gas-station attendant, was gunned down in Mesa, Arizona, by an assailant who had boasted of wanting to kill “ragheads.” Last December, a Hindu American named Sunando Sen was pushed into an oncoming subway train by a woman who explained, “I hate Hindus and Muslims ever since 2001.” Sodhi and Sen, needless to say, weren’t Muslim. They just looked Muslim because they had dark skin.

Even the anti-Muslim epithets that have flourished since 9/11—for instance, “sand n----r”—have a racial connotation. And there’s evidence that Barack Obama’s dark skin is one (though not the only) reason so many Americans still think he’s a Muslim. In 2010, Michigan State University psychologist Spee Kosloff asked supporters of John McCain whether Obama was a Muslim. Then he asked them the same question after they filled out a card in which they listed their race. Once racially triggered, the percentage who called Obama a Muslim jumped 21 points.

You can also glimpse this conflation of religion and race in the demand, which surfaces after every terrorist attack, to single out Muslims for special scrutiny at airports and the like. Often, the politicians and pundits most eager to profile Muslims are the same folks who in the 1980s and 1990s defended the “racial profiling” of blacks. And listening to them, you sometimes get the sense that they think the process would work the same way: just look to see who the Muslims are. In 2011, for instance, Long Island Congressman Peter King suggested that when deciding who police should target as potential terror suspects, a “person’s religious background or ethnicity can be a factor.” But if the problem is Muslims—a billion-person religion with adherents from Malaysia to Mauritania—what does “ethnicity” have to do with it? King then offered a racial analogy: “If I’m told the White Citizens Council, the Ku Klux Klan, is going to attack Harlem, I’d be more suspicious of a white guy walking down around Harlem.” Last year, the atheist writer Sam Harris was even blunter: “We should profile Muslims, or anyone who looks like he or she could conceivably be Muslim.”

At base, the reason it’s so hard for people to accept that the Tsarnaevs are white is because, since America’s founding, being white has meant, both culturally and legally, being “one of us.” And since 9/11, in particular, being Muslim has meant the opposite. As a light-skinned Muslim, Dzhokhar Tsarnaev straddles that divide. But he straddles it in other ways, too. He was a pothead, a devotee of hip-hop, a lifeguard, a high-school wrestler, an aspiring dentist. And yet he became, it appears, a murderer on behalf of a fanatical species of Islam. He’s a type that has reappeared again and again in our history, from every faith and in every shade: an American at war with America, both intimately familiar and frighteningly alien at the same time.