Why has U.S. not been attacked again?

WASHINGTON — Why hasn't there been another terrorist attack on U.S. soil since Sept. 11, 2001?

No one knows for sure, of course. But analysts in and out of government offer three or four theories. Some are comforting, some not.

Better border screening, intelligence sharing and other homeland-security improvements have made it harder for would-be terrorists to operate in the United States.

How much harder is debatable. But every major terrorist attack by Islamic extremists since Sept. 11 has taken place elsewhere: in Europe, the Middle East and Southeast Asia.

Most scholars also agree that although there have been incidents of racial harassment in the past five years, Muslim Americans are better integrated into U.S. society than are their counterparts in Western Europe. With a few exceptions, Osama bin Laden's worldview hasn't caught on here.

In their 2005 book "The Next Attack," former White House officials Daniel Benjamin and Steve Simon cite other possibilities. One, they say, is that the presence of about 130,000 U.S. troops in Iraq is providing plenty of opportunities for jihadists to confront the American superpower directly.

Another — and this fear is shared by many current and former U.S. officials — is that al-Qaida and its affiliates believe they must outdo the carnage of Sept. 11 when they next strike the United States. "Mounting an operation that will kill thousands can take a long time, especially for a group that is averse to failure," Benjamin and Simon wrote.

Sherifa Zuhur, a visiting professor at the U.S. Army War College in Carlisle Barracks, Pa., says al-Qaida, which was surprised by the U.S. reaction after Sept. 11, may be simply waiting for the right moment.

"I wouldn't plan anything unless I was pretty sure I could weather the response," Zuhur said.