re: Reasons
Nobody can start a war just because of beliefs. Convincing facts were needed.
The UN didn't find any WMD before the US invaded Iraq and the worse violations of UN sanctions were only discovered after the US invaded Iraq.
Timing.
From here:
Whatever Bush's specific reason or reasons, the one thing that's so far missing from the record is proof of him looking for a genuine way out of war instead of looking for a way to get it started. Bush wanted war. He just didn't want the war he got.