The most popular of the 9/11 conspiracy theories (as featured in the Loose Change videos, and commonly featured on the Alex Jones radio show) hold that the 9/11 attack was not committed by foreign terrorists but by neo-conservative elements within the US government as a "false flag" operation to curtail our liberties at home and extend our power abroad.
The conspiracy theorists offer numerous pieces of "evidence" to back up their argument, none of which I find particularly valid or compelling (interesting sure, in the paradolia sense). But most significant of all to me is the issue they never address, and to be fair that I've never heard anyone on either side of the debate bring up. The conspirators in our government cannot be both brilliant, masterful, devious plotters and mere months later mind-numbingly stupid and scrupulously honest.
According to the conspiracy theorists, elements at the top of our government pulled off this masterful 9/11 plot as a pretext for invasion of Afghanistan and ultimately Iraq. I believe it reasonable to expect these same conspirators would then turn their skills towards pulling off the justification for the subsequent Iraq war. The US government (from the president, to his cabinet, to the various intelligence agencies) went before its citizens assuring them that Iraq had WMDs that necessitated invasion. The US government went before the UN, and the world, and claimed the same. The US government sent its soldiers to Iraq, toppled the government, and yet found absolutely no WMDs.
If hawkish elements wanted war, wanted it badly enough to deviously murder 3,000 innocent Americans by means of three remotely controlled passenger jets, a cruise missile/drone flown into the Pentagon, controlled demolitions of the Twin Towers, and a controlled demolition of WTC 7, why would they not have produced a single piece of evidence in the months or years after the Iraq invasion that justified the invasion and war? Pulling off a 9/11 conspiracy would be brutally hard work. Pulling off an Iraq war WMD discovery conspiracy would have been far easier. Our government has all manner of WMDs, from the nuclear to the chemical to the biological, with all the powers ascribed to the conspirators under the dominant conspiracy theories, it would have been easy to plant some of our WMDs in Iraq. There was every reason for the same alleged conspirators within the US government to do just that, to prove this was a just war, to prove it to our citizens, to the world, to our soldiers. Proving their case would have secured their position, their politics, their future. And yet no one from the government or military has ever suggested we found anything. Finding nothing was painful, languishing in this debate about whether the war was just was painful. Finding nothing was an embarrassment to the president and the country, and a serious black mark on the various intelligence organizations involved. Finding nothing did us no favors. There was every reason for elements in our government to lie, and yet our government was, apparently, scrupulously honest (about finding nothing).
I do not believe in such inconsistent conspirators; I do not believe this conspiracy.