That is, can Americans really communicate? The word means, literally, "To make common." And at times like this, I wonder if it's possible.

I didn't hear about the Fort Hood shootings until several hours after the news broke, but when I did, much of what I heard wasn't true. Some people told me that the suspect, Army psychiatrist Maj. Nidal Hasan was a "convert" to Islam; others, that he had several Muslim accomplices; still others, that he had links to al Qaeda.

False. False. False.

I got home to find the Internet aflame with vitriol, much of it directed at Islam itself. "Hasan is a BLACK MUSLIM," read a typical blog post. "This was a sleeper Muslim cell terrorist attack ... WITH MORE TO FOLLOW.... Unite AGAINST Islam now people!"

But I also found posts defending Hasan, who was reportedly facing overseas deployment. "They wanted to send him away to kill his own brothers and sisters in Iraq," one post screamed.

"I would have done the same thing!" Finally, others argued that any discussion of Hasan's ethnic or religious background was itself a form of discrimination. "I think giving out the Middle Eastern sounding name of the perpetrator is hate speech," a blogger argued. "No doubt this will give ammunition to patriotic Americans who value national security over diversity." But that's precisely the discussion that we need to have: how to balance security and diversity, unity and freedom.

How can we keep our country safe, but still respect the cultures of its different peoples? How can we join hands as a nation, but remain free as individuals? And it's the same debate we've been having since 1776, when a Congressional committee suggested e pluribus unum - "out of many, one" - for our new national seal. But this discussion - like any real dialogue - requires agreement on a few basic ground rules: civility, reason, and tolerance.

During wartime, to be sure, Americans have often lost sight of these values. Consider attacks on German-Americans during the World War I, when several states banned the speaking of German in schools and on the streets. Or think of the internment of Japanese-Americans - and the confiscation of their property – during World War II. The Internet attacks on "Islam" since Thursday's tragedy lie firmly within this tradition of nativism, bigotry, and hysteria.

The shooter was Muslim, and what else do you need to know? Apparently, not much. But irrationality and bad faith are hardly exclusive to the political right. The Fort Hood shootings have also triggered bouts of left-wing hysteria.

An extreme variation takes the form of the old syllogism, "My enemy's enemy is my friend." You don't like the war in Iraq; neither did Hasan; ergo, he must be OK in your book. Never mind that Hasan gunned down more than three dozen innocents, or that he reportedly defended suicide bombers in Web postings. He's against all the right things, so you're for him.

More commonly, left-wing posters have refused to acknowledge any tension between freedom and security - or any threat to the United States from radical Islam. Hence the bizarre attacks on news organizations for noting Hasan's ethnic and religious background, as if any such information is irrelevant.

It isn't.

There are people living here who want to commit acts of terror, and more than a few of them are radical Muslims. And Texas has seen its fair share.

In 1993, Kuwaiti immigrant Eyad Ismoil was living in Dallas when he was recruited to drive a bomb-laden van into a parking garage beneath the World Trade Center.

Five years later, Lebanese-born Wadih el Hage - Osama bin Laden's personal secretary - was arrested in Tarrant County, Texas, for his involvement in the bombings of two U.S. embassies in Africa. After 9/11, a federal jury convicted five members of a Texas-based Islamic charity of funneling money to terrorists.

And just last month, authorities arrested a 19-year-old Jordanian immigrant, Hosam Smadi, for allegedly attempting to blow up a Dallas skyscraper.

None of that means that Hassan was part of a terrorist conspiracy, of course, or that we should view every Islamic immigrant with suspicion. But it does mean that we have a serious security problem on our hands. And it's simply irrational to deny it.

Indeed, by wishing the problem away, we put off the discussion that we so urgently need. What should we do about potential Islamic terrorists in our midst? How can we protect national security and individual liberty, all at the same time?

These are tough questions, as old as the republic itself. But we'll never get good answers unless we really talk about them. So far, it's not clear that we can.

Jonathan Zimmerman teaches history and education at New York University. He is the author, most recently, of "Small Wonder: The Little Red Schoolhouse in History and Memory."