Many Americans feared that the Sept. 11 attacks were only the beginning.

It seemed possible that Al Qaeda sleeper cells were hidden in American cities. The news media were abuzz with mights and coulds: a smuggled nuclear device detonated in a bustling port; anthrax spewed from a crop-duster across an oblivious city; poison gas cooked up with stolen chlorine. It was conceivable that the first decade of the 21st century would witness the cold-blooded murder of tens of thousands of Americans.

And in fact, some 150,000 people have been murdered in the United States since 9/11. But not one was killed by Al Qaeda. Fourteen were killed by Al Qaeda sympathizers: 13 by an Army psychiatrist at Fort Hood, Tex.; one by a Muslim convert at a military recruiting station in Little Rock, Ark.

How did a determined enemy that had executed so devastating a strike fail so completely to repeat its lethal success?

Certainly the United States’ aggressive response abroad and at home was critical. The Central Intelligence Agency captured Khalid Shaikh Mohammed, whose imagination and management skills had made 9/11 possible, and relentlessly eliminated most of Al Qaeda’s upper ranks. An alert border agent in Chicago turned away a Jordanian who later carried out a huge suicide bombing in Iraq. British and American intelligence foiled a scheme to blow up airliners bound for the United States. Last-minute police work prevented a former Manhattan coffee vendor from carrying out his plan to bomb the New York subway.

But another possibility is worth considering: that Americans and their government, frantic in the wake of 9/11, understandably but hugely overestimated the abilities of Al Qaeda.

In that sense, 9/11 was both a tactical and a psychological victory for Al Qaeda. Terrorism — from the Latin root meaning “to cause to tremble” — persists from century to century because it often achieves its goal: It terrorizes.

Americans reverted to the example of the Cold War, still the shaping experience of most adults in 2001. The Qaeda threat inherited from the communist threat a striking set of parallels: an alien ideology seeking to impose its will by way of a worldwide conspiracy; a suspected web of underground cells inside the United States; concerns about disloyal citizens with a secret allegiance to the enemy.

Even the fear of nuclear weapons was revived, and with it, talk of Al Qaeda as an “existential threat” to the United States.

Such alarm mobilized the government. But it obscured an enormous difference in the scale of the threat: For decades, the Soviet Union had thousands of nuclear missiles aimed at American cities. Even the most generous accounts of Al Qaeda’s capacity posited a single stolen nuclear weapon somehow spirited across a border and detonated.

“We saw Al Qaeda as a pseudo-Soviet successor, and that was misleading,” said Audrey Kurth Cronin, a professor at George Mason University, whose 2009 book, “How Terrorism Ends,” places Al Qaeda in a long history of violent movements that flared and then faded. “By treating it as a kind of state, we gave it a legitimacy it did not deserve.”

Khalid Shaikh Mohammed might deserve the media tagline “mastermind” for 9/11, but he also approved dubious schemes like taking down the Brooklyn Bridge with an acetylene torch. Both the Nigerian who tried to bring down an airliner over Detroit and a former Elizabeth Arden financial analyst who tried to blow up Times Square successfully penetrated American defenses, and both had been trained in explosives abroad. But their bombs fizzled. No scheme involving anything nuclear or biological has gotten beyond idle talk.

For an American, the chance each year of being killed by a terrorist of any ideology anywhere in the world is vanishingly small — about 1 in 3.5 million, according to John Mueller, a political scientist at Ohio State.

Nonetheless, the brazenness of the 9/11 plot and the brilliance and sheer luck of its execution have stood for a decade as an argument that, no matter what any expert says, anything is possible. The attacks inflicted on the American psyche a kind of collective post-traumatic stress disorder, producing at a societal level the hypervigilance that soldiers returning from Iraq and Afghanistan know too well.

“A lot of scenarios that would have been considered far-fetched on Sept. 10 suddenly became plausible,” said Brian Michael Jenkins, a security expert with the RAND Corporation and San Jose State University who has studied terrorism for 40 years. “And there was a fairly relentless message of fear coming out of Washington.”

Between 1970 and 1978, he said, 72 people died in terrorist attacks on American soil — five times the number to die in jihadist attacks since 9/11. “It was a turbulent time, but the republic survived,” Mr. Jenkins said.

The emotional response to 9/11 connected at an intellectual level to a narrative fed both by Al Qaeda and by some outspoken American commentators: that the two sides were engaged in nothing less than a clash of civilizations, a fight to the death over life and liberty. This jihadist narrative magnified any harebrained scheme it touched.

A Muslim who parroted Web screeds about defeating the American enemy has often found a “Qaeda operative” — in fact an undercover F.B.I. agent — suddenly at his service, as Mohamed Osman Mohamud did in Oregon last year. Like other would-be terrorists, Mr. Mohamud was supplied by the F.B.I. with an ersatz bomb, which he tried to detonate at Portland’s Christmas-tree lighting ceremony, setting off only an explosion of media coverage.

Non-Muslims with extreme ideas about overthrowing capitalism or battling racial minorities have been arrested, too, after planning violence. But without the magic ingredient of jihad, connecting to the larger narrative of 9/11, such cases have attracted modest attention.

Al Qaeda or its adherents did mount deadly attacks in Bali, Indonesia; Casablanca, Morocco; Madrid; and London, as well as in Iraq and Afghanistan. Its indiscriminate slaughter of Muslim civilians drove its popularity steadily lower in the Muslim world, but its creed and methods have spread to a few unstable countries. Today, Al Qaeda’s branch in Yemen, where the American-born propagandist Anwar al-Awlaki is hiding, is rated by many experts as a greater threat to the United States than the network’s diminished core in Pakistan. Its two major plots so far aimed at American targets have failed.

After years passed with nothing approaching 9/11, conservative writers and politicians began to warn of a supposed “stealth jihad” to impose shariah, or Islamic law, on the West. The anti-shariah theories fueled last year’s clamor against the Islamic center proposed in Manhattan near ground zero.

Then, just months later, the Arab world was shaken by popular demands not for shariah, but for democracy. Al Qaeda found itself decidedly on the sidelines. In May came the killing of Osama bin Laden. The image of the graying Qaeda founder wielding not a bomb but a remote control capped the impression of a terrorist network in steep decline, made increasingly irrelevant by its own arid doctrine, its brutal tactics and the unpredictable course of history.

Reflecting on those events shortly after moving from C.I.A. director to defense secretary in July, Leon E. Panetta declared that “we’re within reach of strategically defeating Al Qaeda.” Other officials rushed to dispute or qualify Mr. Panetta’s statement, but it marked a striking judgment from a man immersed for years in threat intelligence.

Now, with counterterrorism under the same budget pressure as every other federal program, the blank-check approach of the last decade is certain to change. Professor Cronin, of George Mason, says she thinks that’s not a bad thing. “The fact that we overspent and now have to cut back will force us to be more strategic,” assessing risk with calm resolve and not hype or hysteria, she said.

The emotional response to the unprecedented shock of 9/11 was inevitable, Dr. Cronin said. But now, she said, “it’s time for our nerve endings to calm down.”