A few days before the presidential election, the director of national intelligence, Mike McConnell, told a group of intelligence officials that the new administration could well be tested by a terrorist attack on the homeland in its first year in office. “The World Trade Center was attacked in the first year of President Clinton, and the second attack was in the first year of President Bush,” he said.

President-elect Barack Obama made a similar observation when he told “60 Minutes” that it was important to get a national security team in place “because transition periods are potentially times of vulnerability to a terrorist attack.” During the campaign, Joe Biden warned that “it will not be six months before the world tests Barack Obama like they did John Kennedy.”

Should we be worried? In fact, the probability of a Qaeda attack on the United States is vanishingly small, for the same reasons that for the past seven years the terrorist group has not been able to carry out one.

President Bush and his supporters have often ascribed the absence of a Qaeda attack on the United States to the Iraq war, which supposedly acted as “flypaper” for jihadist terrorists, so instead of fighting them in Boston, America has fought them in Baghdad. Other commentators have said that Al Qaeda is simply biding its time to equal or top 9/11.

The real reasons are more prosaic. First, the American Muslim community has rejected the Qaeda ideological virus. American Muslims have instead overwhelmingly signed up for the American Dream, enjoying higher incomes and educational levels than the average.

Second, though it is hard to prove negatives, there appear to be no Qaeda sleeper cells in the United States. If they do exist, they are so asleep they are comatose. True, in 2003, the F.B.I. arrested Iyman Faris, an Ohio trucker who met with Qaeda leaders in Pakistan after 9/11 and then had a plot to demolish the Brooklyn Bridge with a pair of blowtorches, a deed akin to trying to blow up the Statue of Liberty with a firecracker. But he is an exceptional case. Two years after his arrest, a leaked F.B.I. report concluded, “To date, we have not identified any true ‘sleeper’ agents in the U.S.”

Third, when jihadist terrorists have attacked the United States, they have arrived from outside the country, something that is much harder to do now. The 19 hijackers of 9/11 all came from elsewhere. Ramzi Yousef, the mastermind of the 1993 Trade Center bombing, flew to New York from Pakistan. Today’s no-fly list and other protective measures make entering the country much more difficult.

Fourth, the Bush administration has made Americans safer with measures like the establishment of the National Counterterrorism Center, where officials from different branches of government share information and act on terrorist threats. As a result of such measures, scores of terrorism cases have been aggressively investigated in the United States. But despite the billions of dollars invested in all these efforts and the thousands of men and women who get up every day to hunt for terrorists, the resulting cases have almost never involved concrete terrorist plots or acts.

Of the so-called terrorism cases since 9/11, many have revolved around charges of “material support” for a terrorist group, a vague concept that can encompass almost any dealings with organizations that have at one point engaged in terrorism. And in the cases where a terrorist plot has been alleged, the plans have been more aspirational than realistic.

If Al Qaeda can’t get people into the country, doesn’t have sleeper cells here and is unable to garner support from the American Muslim community, then how does it pull off an attack in the United States? While a small-bore attack may be organized by a Qaeda wannabe at some point, a catastrophic mass-casualty assault anything along the lines of 9/11 is no longer plausible.

This is not to say Al Qaeda is no longer a threat to our interests. It has of course regenerated itself on Pakistan’s border with Afghanistan since 9/11, and as the 2005 attacks on the London subways and the foiled 2006 plot to bring down airliners leaving Heathrow Airport showed, it remains a grave danger to Britain.

In addition, Al Qaeda’s inability to attack the American homeland for the foreseeable future does not then mean that it can’t kill large numbers of American living overseas. If the 2006 “planes plot” had succeeded, British prosecutors say, as many as 1,500 passengers would have died, many of them Americans.

The incoming Obama administration has much to deal with, between managing two wars and the implosion of the financial system and car industry. But the likelihood of a terrorist attack on the United States in its early stages by Al Qaeda is close to zero.