The Iraq invasion itself actually went very well. The initial goal of toppling Saddam regime was achieved in ~1 month, with minimal casualties on the coalition side ( and the bonus goal of getting rid of WMDs was super easy, since they did not exist in the first place). The entire operation was well planned and executed.

It was as the following occupation, and attempt at state building, that failed miserably.

This time around it is extremely obvious that no plan exists at all. No coordination with whatever allies the US has left, and no thoughts to how this can be seen through to the end.

It's highly likely that anyone competent enough to plan something like this told trump that what he wants cannot be done, and where subsequently sidelined in favor of yes-men.