Britains life-insurance policy of joining the US blindly into war, always.
Bit of a sweeping, and incorrect, over-generalization, don't you think?

WW1 - Britain in first, America came in later
WW2 - Ditto
Vietnam - Britain?
Suez - against US wishes (but with France, probably THE stupidest modern war we've ever engaged in )
Falklands - no US involvement (allegedly...)

which leaves us with Korea (1950s / 2010's?), Kosovo, Afghanistan and 2x Iraq.

But back to the issue: I really don't believe that anyone in the UK other than Tony Blair and Sun/Daily Mail readers could have believed the WMD story (and I doubt Blair did). We had weapons inspectors over there telling us there were none, publically resigning because no-one would believe them, eventually being pulled just in time to prevent them getting definitive proof there were none left, thereby ruining the pre-invasion party atmosphere.

It hardly takes FO officials to have worked out that removing Saddam would lead to chaos. Hadn't we just watched Yugoslavia fall apart a few years before without Tito? And Iraq as a nation has always been a fiction, much more so than Yugoslavia ever was.

Yes, Saddam was a terrible ruler -- but replacing a bad rule with likely civil war is not a sufficient justification for an invasion. Two wrongs don't make a right. The whole "regime change" agenda was, and is, to my mind illegitimate, as it defies the very concept of sovereignty which our own nations depend upon. That Saddam was a long-term Western stooge who just went bad on us worsens rather than exonerates our guilt in invading. We should not have intervened in the 1960s, maybe we wouldn't then have "needed" to intervene in the 1990s. If we stop creating monsters, we can stop having to take them down.

But was Blair lying or ignorant? Neither looks good. Personally, I'm verging more towards lying.