One of the things which has always confused me a bit in regards to discussions of just war is when people seem to imply that while it might be just for Country A to resist the attack of Country B, it is unjust for Country C to assist Country A against Country B. It seems to me that thi effectly amounts to arguing that assisting someone in performing a just action is itself unjust, which seems hard to credit at a principled level, though there certainly might be ways you could attempt to assist which would be unjust.
So, for instance, I’ve heard it argued at times that while it might have been just for the South Vietnamese to resist the invasion of the North Vietnamese, it was clearly unjust for the US to get involved in the Vietnam War.
Now, I could see it as being arguable that it was highly unwise for the US to get involved in the war, but it seems hard to understand how it would be unjust, in principle, to assist South Vietnam in defending itself against North Vietnam’s attack.
Is there a principled fashion in which one can argue that it is unjust to assist a country engaged in a just war? If so, what is the argument?
If so, is this a principle which only applies to countries, or are there also examples of personal interactions in which it is just for someone to perform an action, but unjust for another person to assist in that action?