I do not think this is a Roman quote. I think it’s something that was made up in the Viet Nam era. Now, I could be wrong, but I have a couple reasons behind my thinking. First of all, it’s not a Roman sentiment at all. I can’t think of a Roman who would have said that peace was better than just war, let alone very just war. Second of all, where these are legitimate quotes, generally someone, sometime, has attributed it to its author. The Vietnam era is really before the dumbing down of America’s education system led to students not being taught a rigorous curriculum, and so there were lots of people at that time who would have been able to pen such a thought. It certainly fits the US Vietnam kind of thinking more than Roman thinking.

It has been said that things sound smarter in Latin. Writing your political slogan in Latin is a good way to give it more weight, if you’re looking for more weight for your slogan.

I have no hard evidence for you, and I have not read that vast an amount of the Latin corpus.