I recommend reading "Fiasco" by Thomas E. Ricks. You will get the answers to your questions.
havent read it, but am familiar with it.
the iraq intelligence commission report paints a picture of a CIA basically getting alot of things right, but being undercut by WH opinion. of course, the report was blasted by the republicans for being partisan, and a ploy to hurt Bush's chances at re-election.
from what ive all read and seen, Bush got steamrolled by Rumsfeld and Cheney's paranoia, a culture of CIA mistrust was promoted (based off early-90s CIA blunders), and the pressure to pin 9/11 on sadaam lead to some questionable and outright criminal, decision making: Curveball, Chalabi, Hamza, Habbush letter, etc..
the bio-WMD argument is so stupid. no one would have been moved to go into Iraq based off of those charges if not connected to the rest of the allegations. none of my friends who enlisted did so to go get chem weapons. they prob did move them to syria. but we went to get their nukes, which they didnt have. and alot of people knew that. we didnt go for oil (though cheney might have had his own reasons) or to "liberate" them. it was all orchestrated by Rumsfeld's ego, Cheney's paranoia, and Bush's inability to control them.
all of this might have been kept swept under the rug if the invasions and occupation had been handled competently, but we all saw what happened.
i truly cannot understand the thought process of someone getting worked up over Benghazi (obama lied! people died!) and shrug off Iraq, where WH lies actually DID result in mass death.