I'll start from just after the invasion.
1) Troops on the ground discover stores of chemical weapons and processed uranium ore (yellowcake)
2) Troops on the ground immediately secure these stores
3) Troops are deliberately instructed to declare they have not found any such stores
a) The reasoning for this cover-up is to avoid letting the entire region know that there are thousands of tons of these stores in what is essentially a lawless, government-less country.
b) Iraq has previously demonstrated knowledge of advanced uranium metallurgy
as early as 1988. i) Significant stores of enriched uranium were
removed from Iraq in the aftermath of the Gulf War. This indicates that they possessed the infrastructure to enrich uranium.
ii) Related to 3a, Iraq's stores of processed uranium ore and uranium enrichment infrastructure are likewise at rise from the destabilization of the region.
4) The US removes over 500 tonnes of processed uranium ore from Iraq. No one particularly notices or cares.
a) This happens during an election year.
i) Democrat party line cannot mention the facade because it means Bush was right.
ii) Republic party line cannot mention the facade because the area is still unstable (and Bush wasn't up for re-election that year).
5) The current news breaks.
Iraq was in full possession of the infrastructure, means, and materials to manufacture fissile materials and weapons-grade uranium. This infrastructure had been in place since the late '80s, and the presence of so much yellowcake indicated a clear intent to continue development and manufacture (in the same way 500 tons of meth indicates intent to sell meth). US military planners underestimated either the quantity and widespread status of this equipment (and the chemical stuff) and the instability toppling a decades-'stable' dictatorship produces and then collectively **** their pants. Then the cover-up happens.
That covers most of it, I think?
Before the post EDIT: I'm not condoning, supporting, or giving my retroactive approval to the US invasion of Iraq. The point I'm making here is that Bush jumped on a ****ing huge political grenade and we all gave him **** for it for a decade and change when he was actually not wrong.