The Bush Administration developed a policy for invading Iraq, based on some very specific ideas.
Iraq had an active biological weapons program.
The Iraqi government had active ties to al-Qaeda.
The Iraqi government was actively seeking to acquire yellow cake, which can be used to make uranium ore and, ultimately, fuel for a nuclear reactor.
All of these things, of course, were not true.
The allegations that Iraq had an active biological weapons program was based on four sources, three of which were used to support the Iraqi defector who was an unreliable source (CURVEBALL) for the Germans that the American intelligence agencies, almost until the bitter end, did not have direct access to for their own questioning. All four of these sources were discredited or disproven by the American intelligence communities before the invasion, although that did not stop the Bush administration from continuing from use them as reasons to cite the threat of a renewed biological weapons program as a reason to invade Iraq.
Czech counterintelligence officials had claimed that Mohammed Atta, one of the lead 9/11 hijackers, may have met with the Iraqi Consul General from Prague, sometime in April 2001. However, minimal detective work disproved this – but did not keep the Bush administration from continuing to cite the possible connection between Iraq and al-Qaeda. “We just don’t know” seemed to be the motto.
The US Intelligence Community had obtained document that Iraq had attempted to obtain yellowcake in Africa. The CIA had dispatched a qualified senior diplomat to investigate, and he reported back not only that the documents were obvious forgeries, but that the entire scenario was unrealistic and without merit. That did not stop the Bush Administration from continuing to cite the possibility that Iraq might have been trying to secure yellowcake or refine nuclear fuel.
Why do I mention all of this? Because it sound exactly like the argument for BRexit.