Mueller's entire article, which is only a few pages long, is fascinating. Here are some lengthy quotations to get you started:
. . . if it is so easy to pull off an attack and if terrorists are so demonically competent, why have they not done it? Why have they not been sniping at people in shopping centers, collapsing tunnels, poisoning the food supply, cutting electrical lines, derailing trains, blowing up oil pipelines, causing massive traffic jams, or exploiting the countless other vulnerabilities that, according to security experts, could so easily be exploited?
One reasonable explanation is that almost no terrorists exist in the United States and few have the means or the inclination to strike from abroad.
[. . .]
. . . Americans are told -- often by the same people who had once predicted imminent attacks -- that the absence of international terrorist strikes in the United States is owed to the protective measures so hastily and expensively put in place after 9/11. But there is a problem with this argument. True, there have been no terrorist incidents in the United States in the last five years. But nor were there any in the five years before the 9/11 attacks, at a time when the United States was doing much less to protect itself. It would take only one or two guys with a gun or an explosive to terrorize vast numbers of people, as the sniper attacks around Washington, D.C., demonstrated in 2002. Accordingly, the government's protective measures would have to be nearly perfect to thwart all such plans. Given the monumental imperfection of the government's response to Hurricane Katrina, and the debacle of FBI and National Security Agency programs to upgrade their computers to better coordinate intelligence information, that explanation seems far-fetched. Moreover, Israel still experiences terrorism even with a far more extensive security apparatus.
It may well have become more difficult for terrorists to get into the country, but, as thousands demonstrate each day, it is far from impossible. Immigration procedures have been substantially tightened (at considerable cost), and suspicious U.S. border guards have turned away a few likely bad apples. But visitors and immigrants continue to flood the country. There are over 300 million legal entries by foreigners each year, and illegal crossings number between 1,000 and 4,000 a day -- to say nothing of the generous quantities of forbidden substances that the government has been unable to intercept or even detect despite decades of a strenuous and well-funded "war on drugs." Every year, a number of people from Muslim countries -- perhaps hundreds -- are apprehended among the illegal flow from Mexico, and many more probably make it through. Terrorism does not require a large force. And the 9/11 planners, assuming Middle Eastern males would have problems entering the United States legally after the attack, put into motion plans to rely thereafter on non-Arabs with passports from Europe and Southeast Asia.
If al Qaeda operatives are as determined and inventive as assumed, they should be here by now. If they are not yet here, they must not be trying very hard or must be far less dedicated, diabolical, and competent than the common image would suggest.
The rest of the article is filled with similar common sense reasoning about the threat posed by foreign terrorists to the U.S. mainland.