The best and most clarifying thing I’ve read about Magnifica Humanitas comes from Ed Feser, who isn’t trying to explain away anything Pope Leo wrote but rather putting it in context. He’s particularly helpful on just war:

Leo states that “the ‘just war’ theory . . . is now outdated” (192).  Some have read this as an expression of pacifism, which is contrary to the teaching of the Catechism, and of tradition, that war can be justifiable when fought for ends such as self-defense. But that is clearly not what the pope means.  The encyclical goes on to speak of “the principle that armed force should be used only as a last resort in cases of legitimate self-defense”........

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