Among his many, many other flaws, Trump is simply a classic bully. He blusters and threatens and tries to intimidate others into backing down. And if they don't and instead stand up to him, like a classic bully, he's the one who blinks and backs down in many cases, in spite of his tough rhetoric. He's been this way his entire adult life. He learned part of it from the notorious American lawyer Roy Cohn, who served as Senator Joe McCarthy's top assistant in his anti-Communist witch hunt that terrorized Washington DC and a good part of the country in the early-to-mid fifties. There are a lot of similarities between McCarthy and Trump, and one of the connections was Cohn. He told a young Donald Trump to always be aggressive and go on the offensive in dealing with other people, to be a bully, to retaliate for any slights, and that most people are timid and will back down first rather than go through the stress of confrontation.
And so now we have the POTUS threatening the Pope if he doesn't "get on board" and support whatever war Dear Leader has started. It won't work - I doubt a Pope raised in Chicago is going to be intimidated by the blustering and bullying of Donald Trump, a reality show star - but here we are.