Hadrian knew that a wall by itself couldn't keep out the Scots, so he had to have garrisons posted there....Mr. Trump must know that these days electronic surveillance is required, not forty foot walls, unless of course one wants to create jobs. But one would still need reaction forces, and electronic surveillance. Plus, tunnelling is very popular. So the entire wall project is make believe, particularly as there are other financial priorities. He does not have the authority to cut off the Mexican imports and as for U.S. exports to Mexico, these would suffer and there would be a Mexican stand-off, as it were. Still and all, Mr. Trump can say that he did his best to make good on his election promise, come what may. And the same day, Mrs. May came. The whole thing is one big practical joke, tweet by tweet. There is a very good parody in the DT and also the Dutch have done a fantastic job, ending up with an illustration of their big wall keeping out the ocean in their excellent video.