I wouldn't say "justified" but I also wouldn't say "unprovoked."
Iraq having WMDs was a an actual lie, Saddam had nothing to do with Al-Qaeda, and Iraq has never attacked the US. Also, the US's most dangerous enemy in the Middle East was and is Iran. The only power in the region capable of opposing Iran was Iraq, except of course, that after the invasion Iraq is now a hotbed for American hatred, with the Iraqi Shi'ites waiting to take control as soon as possible. So, in all likelihood, a few years, instead of there being one major country controlled by extremist, fundamentalist America hating Shi'ites there will be two countries: Iran and Iraq. Hooray for America.
I highly doubt that Saddam never had any relation with Al-Qaeda. Israel, lest we forget, still exists, and, for the moment at least, can still oppose Iran. Iraq was never friendly with the US anyway, so no love lost there.
I agree that it was badly managed, and probably in the long run not the brightest idea.