Is that true? Maybe.
But anyone who's seen a Tom Clancy movie knows that a handful of bad guys can pull off big conspiracies, especially when they've got a high-level military or government person on board (Clancy writes fiction, but has alot of high-level friends in the military, and knows his stuff).
Moreover, to anyone who knows how covert military operations work, it is obvious that segmentation on a "need-to-know basis", along with deference to command hierarchy, means that a couple of top dogs can call the shots and most people helping won't even know the big picture.
I Can't Hear You
It has now been shown that a handful of people were responsible for willfully ignoring the evidence that Iraq lacked weapons of mass destruction. See, for, example this article. Indeed, Iraq was not the first time the U.S. has ignored or faked intelligence in order to justify war.
The facts are also clear that it was also obvious to U.S. intelligence that 9/11 was going to happen on or around the date it happened.
So how many people would it have taken to ignore the intelligence that hijackers were going to attack? How hard would it have been for a handful of top-level administration officials to stick their fingers in their ears, say "nah nah nah" (like kids do when they don't want to hear what you're saying), and let it happen?
Its Happened Before
As confirmed by a former Italian Prime Minister, an Italian judge, and the former head of Italian counterintelligence, NATO, with the help of U.S. and foreign special forces, carried out terror bombings in Italy and blamed the communists, in order to rally people’s support for their governments in Europe in their fight against communism. As one participant in this formerly-secret program stated: "You had to attack civilians, people, women, children, innocent people, unknown people far removed from any political game. The reason was quite simple. They were supposed to force these people, the Italian public, to turn to the state to ask for greater security."
In the early 1950s, agents of an Israeli terrorist cell operating in Egypt planted bombs in several buildings, including U.S. diplomatic facilities, then left behind "evidence" implicating the Arabs as the culprits (one of the bombs detonated prematurely, allowing the Egyptians to identify the bombers). Israel's Defense Minister was brought down by the scandal, along with the entire Israeli government. See also this confirmation.
But not in the U.S., right? Well, recently declassified documents show that in the 1960's, the American Joint Chiefs of Staff signed off on a plan to blow up AMERICAN airplanes (using an elaborate plan involving the switching of airplanes), and also to commit terrorist acts on American soil, and then to blame it on the Cubans in order to justify an invasion of Cuba. See this ABC news report, the official documents, and watch this interview with the former Washington Investigative Producer for ABC's World News Tonight with Peter Jennings (click link entitled "Joint Chiefs Guilty-Northwoods").
But that was a plan that – while it would have involved a large conspiracy - was never carried out, right? True.
But as shown by this BBC special (which contains interviews with some of the key players), it is probable that America knew of the Japanese plan to attack Pearl Harbor -- down to the exact date of the attack -- and allowed it to happen to justify America's entry into World War II. The case for foreknowledge is even more definitively made by this short essay by a highly-praised historian summarizing some of the key points (the historian, a World War II veteran, actually agreed with this strategy for getting America into the war, and so does not have any axe to grind). According to top WWII scholars, the Pearl Harbor conspiracy involved hundreds of military personnel. And -- most stunning -- the FDR administration took numerous affirmative steps to ensure that the Japanese attack would be successful
These are just a few of many examples of what are called "false flag operations", where governments attack their own people or allies – then blame it on their enemy – in order to justify a war against that enemy. There are many more examples from recent history.
In all of these cases, more than a handful of people were involved in planning, funding and carrying out the attacks. And yet no one spilled the beans or – if someone did – they were not believed.