Although it has faced grave national security threats in the past, the United States has been transformed as never before by the threat of terrorism since the September 11, 2001 terrorist attacks on New York, Pennsylvania and Washington, DC.
The threat of new attacks on American soil and new - albeit widely debated - imperatives in U.S. foreign policy in the Middle East, have the led the nation into costly wars in Afghanistan and Iraq lasting nearly a decade. At home, the country has reconceived its approach to security, creating a new cabinet-level department, the Department of Homeland Security, and granting sweeping new powers to the Federal Bureau of Investigation, the Central Intelligence Agency and other arms of the national security complex.
The declared enemy in this fight is al Qaeda, a stateless organization founded in the late 1980s and motivated by a radical interpretation of Sunni Islam. Al Qaeda is led by Osama bin Laden, the son of a prominent Saudi family, and operates from bases concentrated in Pakistan, Afghanistan, and North Africa.
Critics of the so-called "War on Terror" say that the threat of terrorism - vague and impossible to ever fully extinguish - had been used to justify costly wars (both in terms of deficit expansion and human lives) and abuses of human rights including torture and indefinite detention without trial for alleged enemies believed but not proven to have committed terrorist acts.
Still, the American populace has broadly assented to the changes of the last 10 years, expressing willingness to trade personal liberties and national resources for a sense of safety.
Certainly, the U.S. has not been alone in contending with Islamist terrorism. Al Qaeda and associated groups have killed civilians in attacks in England and Spain and a significant number of attempted attacks on airliners, airports and other sites have either been foiled by authorities while still in planning stages or interrupted in the attempt (as in the case of the attempted Christmas Day, 2009 bombing of a Detroit-bound jet and a man's attempt to set off a car bomb in Times Square, New York in May, 2010).