The president in question was not Donald Trump. It was Ronald Reagan, who in his first year in office raised the possibility that the United States and the Soviet Union could survive an exchange of tactical nuclear weapons. That same year Richard Pipes, Reagan’s director of East European and Soviet affairs on the National Security Council, told The Post he thought the probability of nuclear war was about 40 percent. These remarks sent a signal to Moscow that Reagan was not like those who came before him. He did not want war, but he would not shy from one if provoked.