I'm not American and I opposed the invasion at the time, but it was harder to argue because Saddam's Iraq had very few redeeming qualities, had been waging aggressive wars of its own, and its main role in international trade was as a member of the OPEC cartel.
Whereas Ukraine, for all its problems, is a far more democratic county with far more friends, no history of aggression, and a critical source of food for many nations.
If Russia had invaded Belarus or Tajikistan instead of Ukraine, I doubt there would have been any real consequences.