Scott Ritter has written the Consortium News article Russia, Ukraine & the Law of War: Crime of Aggression.

When it comes to the legal use of force between states, it is considered unimpeachable fact that in accordance with the intent of the United Nations Charter to ban all conflict, there are only two acceptable exceptions. One is an enforcement action to maintain international peace and security authorized by a Security Council resolution passed under Chapter VII of the Charter, which permits the use of force.

The other is the inherent right of individual and collective self-defense, as enshrined in Article 51 of the Charter,…

I have heard him explain this in interviews, but I have not read this article, yet. I want to post it here so I won’t lose track of it.