The huuuugest difference!
The US government claimed that it knew with absolute certainty, based on hard evidence, that Iraq has or was close to having WMD of all three flavors. It never shared that information with the UN weapons inspectors, despite a requirement in 1441 that it do so. The inspectors went in blind, but the US--at least, according to its own propaganda--went in knowing where to look. And has found nothing so far.
Big screwup or Big Lie? Too early to tell.
--Robert