On Sunday, 13 December, a letter is delivered to the White House, starting the most astonishing blackmail of all time:  if President Carter does not submit, the cost will be New York City.  Over the next 36 hours, the action whirls from the Situation Room of the White House to the nuclear laboratories of Los Alamos, from the home of the Israeli Prime Minister to a remote seaside villa, nerve centre of a network global terrorism, thence to the slums, the  glittering discos, the towers of power of New York and the barrenness of the African desert.   Here is a world crisis and the real reactions of men and women to a deadly menace known as the Fifth Horseman.