This story in today's New York Times highlights the problem of the media acting as a watchdog. Back in 2002 and 2003, when the debate over whether or not to invade Iraq was heating up, one of the single strongest pieces of evidence against Saddam Hussein was the testimony of an Iraqi defector going by the code name of "Curveball." Curveball told the CIA and other intelligence agencies that Hussein was continuing to stockpile weapons of mass destruction, specifically mobile bioweapons labs.
Today, Curveball, aka Rafid Ahmed Alwan al-Janabi, admitted that he made up the entire story in order to convince western governments to overthrow Hussein. Along with other Iraqi exiles, al-Janabi's fake testimony led to an invasion under false pretenses (no significant amounts of WMD's were ever discovered).
But here's the big question: did the media fail the public in the run-up to the war? Few media outlets at the time questioned whether the WMD story was true, and few questioned the wisdom of an invasion. Reporters seemingly went along with a herd mentality, accepting whatever information the Bush administration leaked to them without seriously investigating those claims. The media is supposedly a "watchdog" - looking out for the public's interests. Here, they failed miserably in that job.
UPDATE: Donald Rumsfeld admits that without the WMD claim the U.S. "probably" wouldn't have gone to war.