Let’s not split hairs, here. The point of this particular aspect of the doctrine is not to draw an arbitrary line; it’s to demonstrate that one side must determine (as far as is reasonably possible) whether military action will cause evil greater than the circumstance it would seek to change.
That would always be an impossible thing to determine. You would not only have to be able to reasonably predict the number of dead a dictator would kill and the number of lives the war would take, which is always imposible, but have to see all of the after effects.
We know how many people within a million here or so Hitler killed in the few years he had power. How many would his regime have killed? He said his murderous regime would last a thousand years.
WWI not only caused WWII, but the peace terms made after the senseless war was over caused WWII. Not going to war with Hitler early enough without a doubt multiplied the number of dead and the scope of the war. It also caused the suffering of millions in all of eastern Europe that was seized by Stalin’s tyranny. Was Lloyd George immoral in appeasing Hitler? Was it unreasonable to avoid the confrontation as he did? History paints him a fool. His intentions, peace in our time, may have been honorable, but the outcome was a sea of blood.
If Europe stopped Hitler early what would history say of it? It was not justifiable to go to war, just because we thought Hitler might have had bad intentions. Monday morning quarterbacks are always geniuses. Many ofl the Jews in Germany may have been murdered, but an early war would have left those in Poland, France, Italy, etc. alive. No one would have seen the death that would have been stopped by an early war and there would have been a debate on whether it was just.
There truly is a domino theory or principle, but we can not see it. None of us can foresee the consequences of our actions. If Wilson knew what his peace plan would result in he would have not imposed it.
We don’t know what Sadaam would have done if he was unchecked. We know he invaded Kuwait and most certainly would have invaded all the rest of Arabia. Stalin was his idol. How many millions would he and his sons have killed? Whether one approves of the current state of affairs or not, knowing and being able to measure the outcome, or the outcomes of alternatives is impossible.
All of our actions impact the future until the end of time. There are 40,000,000 dead unborn babies in America as a result of the immoral action of a handful of judges. If another country wanted to stop that by force and had a way to do it that would kill only an estimated 4,000,000 would that make a war or a violent revolution to change America’s government just?
What was the evil that made the American Revolutionary War necessary and impossible to avoid? Taxation without representation? To force the King’s government to not look at Americans as second class citizens? To lower the price of tea? Were the Americans so terribly crushed that they had no choice other than war? What gave legitimacy to, or made just, England’s claims over its colonies and its rule imposed by force over the largest empire in history?