On its face, a tweet President Trump sent last week is fairly benign. It doesn’t mock anyone, isn’t personally nasty and hasn’t caused any hair-on-fire controversy. Yet it is powerful in its own way, for it artfully sums up the Trump era from the perspective of the president and his supporters. With a likeness to the Uncle Sam “I want you” poster, the disrupter-in-chief reaffirms in 14 words the belief of Trump Nation that the political establishment, the media, the permanent bureaucracy and yes, the deep state are trying to crush him and them. Critics regard it as fear-mongering, and...