I think it is a valid question and one that every GOP candidate should answer.
Personally, FWIW, I think toppling Saddam was a mistake because (a) he kept the Iranians in check, (b) building a friendly democracy in the Muslim Middle East is a fool’s errand - a super costly and unending burden, and (c) empowering the Iranians increased Sunni support for ISIS.... None of this is to excuse Obama, who made things much, much worse by leaving Iraq precipitously and by supporting the anti-Assad forces in Syria. But just because Obama is a disaster does not in itself prove that the Bush foreign policy was a good idea.
I agree with you and the funny thing to be honest if you look at the whole conflict in Iraq, to an extent we did the dirty work of Iran unknowingly, for we got rid of Saddam, let a Shia led government take over Iraq, and I would think behind the scenes they are thanking us while calling us idiots for doing it.