A side note on #3:
America was threatened. The Zimmerman telegram was Germany’s offer to Mexico to invade America with their support should the US enter the war. They would reclaim territory lost in the Mexican/American war. This isn’t as silly as it sounds today because Mexico’s military was bigger than ours at the time. With German military aid it was not an outlandish idea.
However, Mexico passed. What they knew, and Germany didn’t figure in, was the the Second Amendment. That meant every armed American would be gunning for them. We’d been to Mexico City before, and they remembered that far more than the lost territory they never really governed in the first place.
But once Germany started moving the needle and it looked like they would be able to negotiate a favorable peace, something needed to be done.
Plus no aid the Germans would have sent Mexico would have made it there--or at least not enough to be of sufficient help.
But there was another reason, one which I wonder no one thinks of in today's trouble with Mexico: that country would have had to pacify and assimilate a large Anglo-Protestant population. I wonder why they think they can do this now.