this is very condensed, very simplified.
the most solid believers in the conspiracy theory, the ones who have really done their studies like my friend, believe there was a paradigm shift in the power of the ultra insiders, Illuminati or whatever you want to call them..
they didn't want to kill 25,000, that's why there really were fire drills in the towers just before 9/11 with practice evacuations and there really are multiple confirmations of "mysterious maintenance men" having entire floors closed to the public the week before 9/11 also. their (Ill. pls.) strategy with this whole thing (supposedly) is to make the US appear weak, they want our country to be perceived as vulnerable to the rogue states like DPRK, Iran, the USSR (which does still exist, though not on paper.)
they conjured up this whole "Islamic terror" thing so as to send the US on a "cat herding" mission which is ultimately un-winnable but will however result in long term dramatic improvements to our military system. they wanted to "purge" the US's aging military machinery and have it more advanced than the cold-war era hardware we've been sitting on. this is in preparation for the ultimate show down, the one between the US and China/USSR. all the oil stuff, the islam stuff, it's all distraction and the American public is eating it up. while all this middle east stuff goes on, the union between the US and Canada and Mexico towards forming a new North American Union is in high gear...ie. Bush's comments the other day.