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 is now outdated” (192).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” (197).  It notes that “the Catechism of the Catholic Church speaks of the possibility of legitimate defense by means of military force, which involves demonstrating that certain ‘rigorous conditions of moral legitimacy’ have been met” (footnote 182).  And the complete sentence in which the pope’s controversial words appear reads: “Today, more than ever, without prejudice to the right to self-defense in the strictest sense, it is important to reaffirm that the ‘just war’ theory, which has all too often been used to justify any kind of war, is now outdated” (192).

Feser goes through additional comments by the pope to show that he is speaking of the use of “just war” as an expansive justification for non-defensive wars. I think the allegedly pacifist sentence could have been phrased more clearly, but Feser is persuasive.

He is also helpful in understanding what the pope said about slavery in the encyclical, why he said it, and what he unfortunately (especially in such a long document!) left unsaid.