We know from the Downing Street Memo that Bush was looking for an excuse to invade Iraq.  Here is more evidence:

Bush Ignored Intelligence on Iraqi Weapons, Says Ex-CIA Officer
By Dan Glaister
The Guardian UK

Monday 24 April 2006

A former leading CIA official said yesterday that the White House deliberately
  ignored intelligence that showed that there were no weapons of mass destruction
  in the run-up to the war in Iraq.

Tyler Drumheller, who was once the highest-ranking CIA officer in Europe, told
  CBS’s 60 Minutes programme that the White House shifted its focus to regime
  change in the months before the invasion.

"The policy was set. The war in Iraq was coming and they were looking
  for intelligence to fit into the policy," Mr Drumheller said.

Meanwhile a leaked Pentagon document showed that Donald Rumsfeld, the defence
  secretary, is pressing ahead with plans to reshape the armed forces despite
  recent criticism of his stewardship from several retired military officers.
  Plans approved last month by Mr Rumsfeld and leaked to the Washington Post revealed
  the increasing use of special forces and an increased role for the military
  in areas that have been the domain of the CIA.

Mr Drumheller, who had a senior role in the run-up to the war, told 60 Minutes
  that the CIA provided the White House with information from Iraq’s then foreign
  minister, Naji Sabri, who had reportedly made a deal with the US.

"[The source] told us that there were no active weapons of mass destruction
  programmes," Mr Drumheller said. He said that the then-CIA director, George
  Tenet, passed the information on to George Bush, Dick Cheney, the vice-president,
  and other senior officials, who were initially excited. But that changed, he
  said.

"The [White House] group that was dealing with preparation for the Iraq
  war came back and said they were no longer interested. And we said ‘Well, what
  about the intel?’ And they said ‘Well, this isn’t about intel anymore. This
  is about regime change.’"

Mr Drumheller said the decision to invade Iraq would be remembered as a grave
  mistake. "It just sticks in my craw every time I hear them say it’s an
  intelligence failure … This was a policy failure … I think, over time, people
  will look back on this and see this is going to be one of the great, I think,
  policy mistakes of all time," he said.