You claim to be Catholic which officially teaches the acceptability of defensive war. But I suppose that's never stopped you from just making whatever you want up.
Anyway, in regards to my thoughts on the OP; we all agree that violence is horrible. However, in certain circumstances, I think to avoid it can be even greater an evil. Should Brittan have not fulfilled her agreed obligations to Belgium in WWI? Should Nazi Germany have been able to invade Poland with no reprisal from the Allies? Are we to allow such aggression to be unopposed out of a philosophical and moral diastase for violence? (no matter how understandable) I personally don't think so. I'm sympathetic, but I just don't think total pacifism is a realistic stance. Especially if it means essentially piggybacking on the protection of those who are willing to fight.
I don't believe in the Pope's infallibility. So, if the Pope claims the acceptability of defensive war, I claim that he's quite wrong. That's not what Jesus teaches.
As for the WW2: well, the Germans invaded Poland. so what? A Christian is supposed not to react, because the Nazis were the perpetrators, not the Polish.
there is an instrument which is more powerful than violence: the word. When the Nazis invaded Rome, the Pope did not say anything about that.
The Nazis arrested and deported lots of Jews from the Roman Jewish District. The Pope didn't say anything. even if he secretly saved lots of Jews, he was still a coward. Because he didn't use the word to protest against Hitler.