It was viewed as illegitimate inside the US too.
You’re recollection of events is clearly skewed. Something like 80% of the population approved of it at first. Meanwhile there were protests in the millions of people around the world against it.
https://en.wikipedia.org/wiki/Public_opinion_in_the_United_States_on_the_invasion_of_Iraq
A Gallup poll made on behalf of CNN and USA Today concluded that 79% of Americans thought the Iraq War was justified, with or without conclusive evidence of illegal weapons.
I clearly stated “at first”. Mind you by the end of his term a majority still thought it was the right thing to do.