As Jake said, there IS no agreed definition of "illegal war", so anyone who imagines there is is simply talking through a hole in his hat.
If "killing of innocent civilians" was an international crime, President Mitterand should have been tried for the sinking of the Rainbow Warrior in New Zealand. One victim or 100,000...what's the difference in terms of criminality?
In addition, I have no doubt but that "innocent civilians" died when NATO and Russia went into Kosovo in the nineties. (Despite the fact that no UN sanction was given for THAT incursion either, I don't recall anyone screaming "Illegal war!" then; nor can I recall just how much oil we got for doing so!)
France got involved in Indo-China and China got involved in the Korean War, so each and every one of the five permanent members of the UN Security Council got into a violent situation when they felt it suited them to do so and let "illegality" go to blazes!