there are theories that fdr knew about the attack on pearl harbor and let it happen so he had an excuse to join the war. i'm not saying it's true, i'm saying it's a theory.

as for how certain the wmds, check this out:
some selected text from http://billmon.org.v.sabren.com/archives/000172.html

Simply stated, there is no doubt that Saddam Hussein now has weapons of mass destruction.
[nocando] Cheney
Speech to VFW National Convention
August 26, 2002

Our intelligence officials estimate that Saddam Hussein had the materials to produce as much as 500 tons of sarin, mustard and VX nerve agent.
George W. Bush
State of the Union Address
January 28, 2003

Intelligence gathered by this and other governments leaves no doubt that the Iraq regime continues to possess and conceal some of the most lethal weapons ever devised.
George W. Bush
Address to the Nation
March 17, 2003

I'm absolutely sure that there are weapons of mass destruction there and the evidence will be forthcoming. We're just getting it just now.
Colin Powell
Remarks to Reporters
May 4, 2003

I just don't know whether it was all destroyed years ago -- I mean, there's no question that there were chemical weapons years ago -- whether they were destroyed right before the war, (or) whether they're still hidden.
Maj. Gen. David Petraeus, Commander 101st Airborne
Press Briefing
May 13, 2003

Do I think we're going to find something?? Yeah, I kind of do, because I think there's a lot of information out there."
Maj. Gen. Keith Dayton, Defense Intelligence Agency
Press Conference
May 30, 2003

It was a surprise to me then ? it remains a surprise to me now ? that we have not uncovered weapons, as you say, in some of the forward dispersal sites. Believe me, it's not for lack of trying. We've been to virtually every ammunition supply point between the Kuwaiti border and Baghdad, but they're simply not there.
Lt. Gen. James Conway, 1st Marine Expeditionary Force
Press Interview
May 30, 2003