[colonelanguz]

I am sympathetic to the point that you are making and hate to go all legal-positivist, but an illegal war is a war that isn't legal under international law. Defending your country from an invasion is like the gold standard of legality. It sits in contradistinction to, e.g., wars of aggression, which are what they sound like. To be a legal war where you're not defending against an invasion, it's necessary to have either a UNSC resolution supporting the war or a compelling self-defense/preemptive justification.

For the 2003 invasion of Iraq, people argue that it was illegal because there was no UNSC resolution. The U.S. and U.K. argued that it was authorized under the UNSC resolution that blessed the first U.s. invasion of Iraq. I think that sounds like nonsense, and particularly when you factor in that the actual pretenses for the 2003 invasion were false and probably fabricated by the aggressing parties—it seems like not a stretch to call it an illegal war.

Obviously the 2003 Iraq Invasion doesn't itself get you to OP's 4 million body count, but I hope this is a helpful start.

[runawaybottle]

Without getting too much into international law, which is mostly laughed at, evidently, sometimes I think it’s just better to see some video:

https://youtu.be/1Z3f_p_7OeE

It was very illegal to put it quite frankly.

[colonelanguz]

100%. Not that this means anything but I'm a JD and my JD girlfriend specializes in international law and we agree it's basically a joke. But hell yeah it was illegal.