Started by the French no less. Their hatred for America knows no bounds. The article is quite detailed, below is a portion of it.
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http://www.snopes.com/rumors/pentagon.htm
The notion that the Pentagon was not damaged by terrorists who hijacked American Airlines Flight 77 (a Boeing 757) and crashed it into the military office complex, but that the whole affair was staged by the U.S. government, has been promulgated by French author Thierry Meyssan in his book, The Frightening Fraud. Meyssan offers no real explanation for what did cause the extensive damage to the Pentagon, asserting only that Flight 77 did not exist, no plane crashed into the Pengaton, and that "the American government is lying."
Unfortunately, the appeal of conspiracy theories has resulted in widespread dissemination of Meyssan's "theory" in France and the USA, particularly in web sites that mirror his work. As Le Nouvel Observateur noted: "This theory suits everyone - there are no Islamic extremists and everyone is happy. It eliminates reality."
The text cited in the example above comes from a Hunt the Boeing! And test your perceptions! web site, one of the English-language mirrors of Meyssan's claims, where readers are invited to ponder a series of questions about why photographs of the damaged Pentagon seemingly show no evidence of a crashed airplane.