Actually, it was presented simultaneously as the need to uphold UN resolutions AND as the need to maintain national security regardless of UN resolutions. In other words, the administration's reasons were simultaneously out of concern and lack of concern for the UN.
And yes, that's LITERALLY simultaneously. Two administration officials (i.e. officially representing administration policy) in two different speeches in the same morning said "It's a UN issue, the US can't take unilateral action" and "It's a US national security issue, the UN is irrelevant." Again, those were official administration statements given within an hour of each other. So no, it was never "constantly" presented as that...in fact, there was nothing constant about the administration's presentation.
But the thing really missing from the discussion that makes my point is: there's an executive finding, written sometime around 2001-2002, that says "The greatest threat to national security is terrorists with WMDs." Every other justification the administration ever gave was bull sh--...the real reason was that, as a matter of policy, this administration tied terrorism to Iraq via the logic of the international issue of counter-proliferation...which very nearlty makes Iraqi non-compliance with UN resolutions an anti-terrorism issue and justifies the whole invasion...pretty much makes it a foregone conclusion, really.
And it does. It provides perfect justification. If you accept the a priori assumption that terrorist WMDs were a greater threat to national security than pissing off the entire world with the unprovoked unilateral invasion of a soverign nation...