Bush lied

One of the core arguments made by the Bush Administration to justify the invasion of Iraq was the supposed links between Saddam Hussein and Al Qaeda. In February 2003, President Bush was claiming that

Saddam Hussein has longstanding, direct and continuing ties to terrorist networks. Senior members of Iraqi intelligence and al Qaeda have met at least eight times since the early 1990s. Iraq has sent bomb-making and document forgery experts to work with al Qaeda. Iraq has also provided al Qaeda with chemical and biological weapons training.

and that

We also know that Iraq is harboring a terrorist network headed by a senior al Qaeda terrorist planner. This network runs a poison and explosive training camp in northeast Iraq, and many of its leaders are known to be in Baghdad.

Even after the war, these claims continued. When the September 11th commission reported that there were no such links last year, Bush said

"The reason I keep insisting that there was a relationship between Iraq and Saddam and al Qaeda: because there was a relationship between Iraq and al Qaeda," Bush said after a Cabinet meeting. As evidence, he cited Iraqi intelligence officers' meeting with bin Laden in Sudan. "There's numerous contacts between the two," Bush said.

In recent days these claims have been undermined by evidence that the Bush Administration knew that their major source - Ibn al-Shaykh al-Libi, a high-level member of Al Qaeda - "was intentionally misleading [his] debriefers" (though "torturers" would be a better term, given that he had been rendered to Egypt). But now there is an absolute smoking gun: the President's Daily Intelligence Briefing of September 21st, 2001:

Ten days after the September 11, 2001, terrorist attacks on the World Trade Center and the Pentagon, President Bush was told in a highly classified briefing that the U.S. intelligence community had no evidence linking the Iraqi regime of Saddam Hussein to the attacks and that there was scant credible evidence that Iraq had any significant collaborative ties with Al Qaeda, according to government records and current and former officials with firsthand knowledge of the matter.

[...]

One of the more intriguing things that Bush was told during the briefing was that the few credible reports of contacts between Iraq and Al Qaeda involved attempts by Saddam Hussein to monitor the terrorist group. Saddam viewed Al Qaeda as well as other theocratic radical Islamist organizations as a potential threat to his secular regime. At one point, analysts believed, Saddam considered infiltrating the ranks of Al Qaeda with Iraqi nationals or even Iraqi intelligence operatives to learn more about its inner workings, according to records and sources.

So, the whole time Bush was claiming in public that Saddam was in bed with Osama, he knew damn well that what he was saying was false. He lied - and at least 2000 American soldiers and 30,000 Iraqi civilians are dead as a result.