Chalmers Johnson has written a book titled Blowback: The Costs and Consequences of American Empire that tries to find the answer to this anomaly. He focuses on evaluating the extent to which American foreign policy inspires terrorism. Most of the book is based around supporting his central assertion of something called blowback: "a term the CIA invented to describe the likelihood that [American] covert operations in other people's countries would result in retaliations against Americans, civilian and military, at home and abroad."
For Johnson it is no mystery. Terrorists generally don't dislike America for what it represents (material wealth and democracy), but for what the American government does in foreign nations.
Indeed, one of Osama bin Laden's grievances was the stationing of American troops in Saudi Arabia. Yet the US did not withdraw its troops until 2003, despite repeated terrorist attacks (that is, blowback) throughout the 1990s. Somehow neither Democrats nor Republicans understood the message from the terrorists: we don't want you interfering in our internal affairs, leave us alone. The good that came from eventually withdrawing American troops from Saudi Arabia has now been negated by waging two unnecessary and constitutionally suspect wars in the heart of the Middle East. Again, one observes the common American government practice of meddling in other nations.
A consistent theme of American foreign policy has been picking winners - usually incorrectly. In Afghanistan, the Americans funded the Taliban in the 1980s, but then changed their mind after September 11 and came back and supported the opposition Northern Alliance. Picking winners is a favourite habit of governments in the economic realm, and it is also evident in foreign policy. But picking winners also means the losers begin to resent you.
While distaste for American values or religious fervour might be a propaganda tool terrorists use to motivate followers, the underlying tension is created by foreign policy actions taken by the American government, starting with the CIA's overthrow of the Iranian leader in 1953. Since that time, American policy has become increasingly interventionist, and the CIA has engaged in numerous clandestine operations that many Americans would be appalled of, if they knew what went on in their name.
Johnson wrote the first edition of his book before the September 11 attacks. In it, he correctly predicted acts of retaliation upon US interests. His argument is that everyone reaps what they sow; the worst blowback from the 20th century, and more recently from Iraq and Afghanistan, is yet to come.
The thousands of innocent Iraqi civilians that have been killed due to American "friendly fire" have families. Even if we in the West forget these Iraqi deaths, their families won't. The terrorists will capitalise on the resentment against Americans to gain financial support. This means Americans should prepare for the hatred their government has recently generated in the Middle East. It means there will almost certainly be another major terrorist attack on American soil 5-10 years down the track.
Sadly, more American citizens have died in Iraq than were killed on September 11. It is time America reverted to a humble foreign policy that focuses on securing American democracy and liberty, before it justifies billions of dollars and thousands of lives by citing false security concerns or the need to install democracy by force in other nations. Blowback is real, and the quicker Americans understand this, the quicker America can stop being a target for terrorism.