It sounds like something out of a novel: a cell of heavily armed German extremists plotting to overthrow the elected government and elevate a man called Prince Heinrich XIII to the throne of a new Teutonic monarchy. On Wednesday, German police arrested 25 people attempting to do exactly that — including a former member of parliament from Alternatives for Deutschland (AfD), a far-right anti-immigrant faction. The plot originated out of a movement called the Reichsbürger — literally, “Reich citizens.” They believe that every German state since World War I has been illegitimate, a corporation rather than an authentic government, and...