On an October afternoon before the 2016 election, a huge banner was unfurled from the Manhattan Bridge in New York City: Vladimir V. Putin against a Russian-flag background, and the unlikely word “Peacemaker” below. It was a daredevil happy birthday to the Russian president, who was turning 64. In November, shortly after Donald J. Trump eked out a victory that Moscow had worked to assist, an even bigger banner appeared, this time on the Arlington Memorial Bridge in Washington: the face of President Barack Obama and “Goodbye Murderer” in big red letters. Police never identified who had hung the banners,...