“United 93,” which opens the Tribeca Film Festival tonight, tells the story of the first victory of Americans over our 21st-century terrorist enemies with a gritty realism that many viewers will find difficult to watch. Real footage of the September 11, 2001, attack is mixed with dramatization of the actions taken (and not taken) at air traffic control, at the Federal Aviation Administration, at the headquarters of America’s air defense, and, most important, on board United Airlines Flight 93.

The film seems to take place in real time, moving slowly toward the events that transformed an ordinary day in September into an epochal day called “9/11.” The first half of the film is perhaps the most difficult to watch, as pilots and passengers, air-traffic controllers and bureaucrats begin the morning of September 11, 2001, with the mundane rituals of air travel. Flights are delayed, vacation plans are discussed, flight attendants take breakfast orders, and one passenger just barely makes it onto his flight. There’s not a hint of suspense. We know exactly what is going to happen. What we feel instead is an emotion that I do not remember ever feeling so strongly in a movie theater. That feeling is called dread.

The attacks are already under way when Flight 93 takes off. We know this, but almost nobody in the film knows it yet. There are suspicious communications from one or two other commercial airliners already in flight. The vigilant air traffic controllers pass the news of a possible problem up the chain of command, but only the terrorists themselves know that four elaborate, murderous suicide missions are under way. It is more than a little disturbing for a member of the audience to share this knowledge with the terrorists.

When Flight 93 passes near Manhattan, we get a glimpse of the soon-to-be-attacked World Trade Center out of one of the passengers’ windows. The two towers press up into the sky, incredibly taller than anyone can remember them. For the passengers of Flight 93, they are a tourist attraction. For the audience, they are a premonition of murder.

The first plane strikes the World Trade Center while no one is watching. The FAA and air defense see the tower smolder, uncertain whether this is an accident or an attack. Then we watch with them as the second plane makes that evil turn into the World Trade Center. The image is still powerful enough to strike like a spear to the heart.

Onboard Flight 93, the terrorists pray and hesitate. The ringleader, the man who trained to pilot a plane without learning how to land one, delays his fellow killers so long that one almost wonders if he will go through with the scheme. It is worth paying close attention to this scene because it is the best depiction of what the experts tell us are the warning signs of an imminent terrorist strike – a collection of young Muslim men from the Middle East, nervous, sweating, and praying.

Of course, not many of us knew what to look for before September 11, and so the terrorists capture the plane with disturbing ease. The passengers are herded into the back of the plane, and the pilot and co-pilot are unceremoniously butchered.

The various levels of government fall into a heartbreaking paralysis in the face of the attacks. The air traffic controllers are portrayed as alert and highly competent, but things fall apart rapidly as one climbs up the power structure. As portrayed in the film, the military response is negligible and negligent, with air defense commanders unable to marshal the resources or get authorization to strike against the weaponized airliners.

I have heard it said that the 21st century began on September 11, and I would go so far as to say it began on United Flight 93. When the passengers reach loved ones on air phones and learn for the first time that the hijacked planes are being turned into murder machines, something changes. “United 93” refuses to mythologize this moment, and the shift is subtle. “This is a suicide mission,” one of the passengers says. The next thing that happens needs no mythologizing at all – among a small group of men, this realization transforms their fear into resolve. They don’t intend to die, and they don’t intend to be used as fodder for the terrorists’ plan.

The passenger revolt is preceded by enough doubt and hesitation that when it finally comes, we are relieved. No doubt this is meant to mirror the images of the hesitant terrorists at the start of the film. Indeed, we see both passengers and terrorists praying just before the counterstrike. But it would take the worst kind of moral relativist to see these prayers and the actions that follow them as equivalent. I am confident no one watching “United 93” will make that mistake.

When the group of men on the plane – who the best evidence we have tells us included Thomas Burnett, Jeremy Glick, and Mark Bingham – launch their assault on the terrorists, they inaugurate a new era. “Make us a pack, not a herd” was a phrase uttered often in the immediate aftermath of the attacks. “United 93” shows us how quickly these brave men made themselves a pack of heroes.

September 11 was a disaster for radical Islam. For the radical Muslims of Al Qaeda and its fellow travelers, it marked the beginning of the failure of their dream simultaneously to launch a worldwide Islamic revolution and break the spirit of Western civilization. Even the most murderous and elaborate attack against Americans on American soil could not accomplish these goals.

For the particular group of Muslim terrorists portrayed in “United 93,” September 11 was an even more personal failure. The men who hijack United Airlines Flight 93 seek to convert the airplane into a missile to be used against our U.S. Capitol building, just as two other planes were used to destroy the World Trade Center and one to attack the Pentagon. Instead, because a few American men learned so quickly the lesson of September 11, Flight 93 went down in a field in western Pennsylvania.

Make no mistake. “United 93” is a difficult, frustrating, heartrending movie to watch. But it is worth it if it helps preserve in our minds the lesson United Flight 93’s passengers died teaching us.