One of the standard bearers of political Islam finds itself square in the White House's crosshairs. The Trump administration confirmed that it is weighing whether to designate the Muslim Brotherhood, an organization that is prominent in politics and society throughout the Sunni world, as a foreign terrorist organization. If the United States were to add the Brotherhood to its list, it would join Russia, Bahrain, Saudi Arabia, Syria, Egypt and the United Arab Emirates in doing so. Such a move threatens to open a can of worms for the United States, however, not least because the group has so many incarnations -- in so many countries -- that it defies easy designation. More important, though, are the political difficulties Washington will create for itself in taking a firm stand against the Brotherhood: For while prominent foes of the group in Cairo, Riyadh and elsewhere will laud the move, the United States...