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,

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