By 9 p.m. on Thursday, the spacious West Wing corner office of National Security Advisor Gen. H.R. McMaster sat empty, the door open, the blinds undrawn, pale blue papers on the desk bearing messages from world capitals, unanswered. For White House staffers already on edge after a week of tumultuous personnel and policy change driven by President Donald Trump, the scene represented an ominous calm before the latest, most worrying disruption at the top of American government. Two hours earlier, Trump had announced on Twitter that he would be replacing McMaster, a temperate, soldier-scholar who had closely hewed to the traditional role of “honest broker” in reconciling the often competing agendas of the CIA, the State Department and the Pentagon, among other foreign and national security agencies in the U.S.