> the US did not declare their invasion of Iraq to be a war.
That's a purely domestic issue within US law. The full-scale invasion of another country constitutes "war" for the purposes of international law.
The Iraq War is one of the clearest examples of an illegal war of aggression in modern times.
The US faced no serious threat from Iraq. Even if one were to believe all the claims about WMD (which were made in bad faith), Iraq would still not be an imminent threat to the US. Defensive wars have to be in response to an imminent, concrete threat, like an enemy force massing on the border. The mere fact that a hostile country possesses weapons does not justify a pre-emptive strike. If it did, the ban on aggressive war would be meaningless.
Of course, as is now known from leaked memos, the entire WMD issue was viewed by the Bush administration as a convenient pretext for invading Iraq.[0] Many people (myself included) already saw through the WMD propaganda back in 2002-2003, despite it being relentlessly hyped by major media (from the New York Times on down). I personally was surprised, however, by just how cynical the WMD claims of the Bush administration later turned out to be: for every claim they confidently made in public, they knew the evidence was thin to non-existent.
If the invasion of Iraq - a non-defensive war, sold using a dishonest media campaign, waged without UNSC authorization by the world's most powerful country against a comparatively tiny country - isn't an illegal war of aggression, then nothing is.
0. Downing Street Memo: https://en.wikipedia.org/wiki/Downing_Street_memo