It's bothersome because yet again, Trump is lying, and yet again, he shows his willingness to order murder for no other reason than to make himself look tough.
But the order doesn't make any sense. The order isn't an order, it's a memo, and it wasn't even signed by Trump, but by John Kelly, who is not even in the military chain of command. As currently constituted, it has no legal or military force whatsoever. Nobody in the Army has any obligation to follow it. Mattis himself has already said that the mission hasn't changed.
Speaking of which, what actually is that mission? Well, the troops finished putting up most of their concertina already. Since they don't have enough C-wire to create an obstacle around, really, much of anything, it looks pretty funny. There's already a picture going around of where the wire ends around Laredo, about a mile out of town. Furthermore, doctrine states, and current military rule of thumb emphasizes, that "an obstacle not covered by fires is a nuisance". And that is still what it is. The Army detachments are not stationed behind this ostentatious and very pointless C-wire. They are not covering it with fires. That would be preposterous in this scenario. Instead, after creating this silly photo op, they have gone back to their bases and bivouacs to perform maintenance for Border Patrol vehicles and play Spades. That is, after all, what they're actually for. Much like the photo op was (failed) theater for the midterm election, this "order" is theater for after the election. Trump wants something that will create a media splash. Whether he actually wanted it to be a real order but had it watered down to a meaningless "memo", or meant it from the start as pure theater, that is what it is.
