The soul-searing spectacle of a September morning changed the way America looks at the world. Now a year of war, of ultimatum, of overwhelming power is changing the way the world sees America.
"Either you are with us, or you are with the terrorists," President Bush told other governments in the first days of national shock a year ago, when Congress rushed through $40 billion for a war on terrorism and the U.S. military soon embarked, in Afghanistan, on its longest combat engagement since wading ashore in Vietnam almost four decades earlier.
For America, the counterstrike to Sept. 11 shattered old barriers and opened dangerous new horizons.
It landed U.S. forces in former Soviet territory for the first time, as a U.S.-Russian partnership grew stronger. It put American military teams into unfamiliar combat zones on the fringes of the Islamic world. And it produced a sharper U.S. tilt toward Israel.
Along the way, the forceful moves made enemies and complicated the support of friends. A wider war would mean still deeper complications.
"By confronting evil and lawless regimes, we do not create a problem. We reveal a problem," Bush told graduating West Point cadets nine months after Sept. 11.
What confronting Iraq's regime would create - a review of world opinion makes clear - is a stark image of America as enforcer of the status quo: an exclusive "nuclear club" of nations, a protective relationship with Arabian oil princes, an Israel of unrivaled superiority in its neighborhood.
The risk in the challenging times ahead is the "blowback" - the unforeseen, just as a generation ago America's Afghan proxy war against the Soviets helped produce an unintended consequence named Osama bin Laden.
It was no surprise when old friends like Britain and Canada rushed to the United States' side.
More striking was the solidarity shown by Russia, in words and quiet support, coming barely a decade after the two countries were locked in nuclear standoff.
President Vladimir Putin's backing for the American campaign reflected, in part, Moscow's desire to draw closer as an economic partner, in part its desire to cast its own war with Chechen separatists in the same light. The Putin embrace allowed the long arm of the Pentagon to base troops in the former Soviet states of Kyrgyzstan and Uzbekistan, on Afghanistan's northern border.
Even more daringly, Pakistan's military ruler, Gen. Pervez Musharraf, allowed American soldiers onto his Muslim soil, turned against onetime friends in Afghanistan and threw his own forces into the hunt for al-Qaida and Taliban holdouts.
Numbers tell the story of America's changed role: In weeks, its military strength in the region quadrupled to 60,000 soldiers, Marines, sailors and airmen.
Elsewhere, Washington has sent Army trainers to the Philippines, whose government is fighting Muslim separatists; to former Soviet Georgia, where Muslim guerrillas control a mountain region; and to Yemen, a sometime base for Islamist terrorists.
The "with us or against us" campaign found help at other international levels as well, in exchanges of police intelligence, in the tracing of terrorist funds, in arrests of suspects - more than 1,300 in 70 countries.
In central Asia and elsewhere, America found new friends. But as the days stretched into months, as the freeze-frames of a morning's horror gave way to scenes of havoc in places called Kunduz, Kabul and Kandahar, sympathy began to fray.
"Today, the U.S. is the victim," a tough critic of the Islamists, Pakistani commentator Pervez Hoodbhoy, wrote after Sept. 11. "But the carpet-bombing of Afghanistan will cause it to squander the huge swell of sympathy in its favor the world over."
The eventual air campaign was not carpet-bombing, but it was devastating. History's most powerful nation dropped more than 18,000 bombs and missiles on one of the world's poorest lands. Across the Muslim world, and beyond, the sight angered millions.
Muslims were also angered by the secret roundups of U.S. Arabs and a plan to fingerprint Middle Eastern visitors.
A Gallup poll three months into the war found that esteem for America was as low in Saudi Arabia as in Iran - just 15 percent in both. Even in Kuwait, freed from Iraqi conquest by U.S. troops 11 years earlier, only 28 percent "favored" America.
Even beyond the Muslim belt stretching from Africa to east Asia, the mood of September - the human compassion for innocent lives taken - gave way to new emotions, new concerns.
In Europe, although governments stood by Washington and supplied at least token special forces to the Afghan war, popular criticism sharpened.
The horrors of Sept. 11 awoke Americans to a wider world of threats and hatred for their powerful nation.
"Stereotypes of the United States as arrogant, self-indulgent, hypocritical, inattentive, and unwilling or unable to engage in cross-cultural dialogue are pervasive and deeply rooted," said a U.S. Council of Foreign Relations study.
The many mornings since Sept. 11 have awakened many Americans to more than that - to the interdependence of an ever-smaller world, to their own dependence, to their vulnerability.