The Government Let It Happen on Purpose
Some conspiracy theorists subscribe to the idea that the U.S. government did not plan the attacks, but was well aware of the terrorist plot and did nothing to stop it. This is different from the idea that certain officials were warned of a possible attack and didn't give the warnings enough credence to act. Indeed, these theorists believe that officials "let it happen on purpose" (shortened as LIHOP). According to such theories, the U.S. officials consciously allowed the Sept. 11th attacks to go forward, thereby abetting the violent deaths of almost 3,000 people and, as far as they could have predicted, potentially more.
So why would these supposedly sinister, powerful officials consciously condone such horror and allow it to happen? Conspiracy theorists speculate that the officials, including members of the Bush administration, wanted an excuse to invade Afghanistan and Iraq. Theorists point to a document from 2000 by Project for the New American Century (PNAC), a neoconservative think tank. The document states that in order for the military to "fight and decisively win multiple, simultaneous major theatre wars," it must go through a transformation which is "likely to be a long one, absent some catastrophic event -- like a new Pearl Harbor" [source: McConnachie].
Whether one believes that the Bush administration exploited the Sept. 11th attacks for its defense agenda is different from believing that the administration previously knew of and allowed the attacks. Many doubt these officials would be capable of such evil complacence.