An admirer, friend and disciple of William F. Buckley, the conservative author and television commentator, he held several posts in Mr. Bush’s administration, including as an adviser to the U.S. ambassador to the United Nations — John R. Bolton, with whom he would later clash on Iran policy. He also kept teaching at Duke University during his tenure, and said that he would continue in that role.
Mr. Hook joined the Trump administration in early 2017, under Secretary of State Rex Tillerson, as director of the State Department’s office of policy planning. Mr. Tillerson gave that office increased influence over all the department’s initiatives, and Mr. Hook became one of his closest advisers.
It was Mr. Tillerson who introduced him to Jared Kushner, the president’s son-in-law and senior adviser. The White House at the time had, in essence, no foreign policy at all — just a series of Mr. Trump’s pronouncements. Mr. Kushner said in an interview on Wednesday that Mr. Hook “helped us develop an approach to the world that we are now executing.”
“Brian was the traveling companion who could help guide some very fluid meetings with foreign leaders,” giving updates on Iran to King Salman of Saudi Arabia, among others. And it was Mr. Hook who had the idea of having Twitter posts sent out from Mr. Trump’s account in Farsi, to address the Iranians.
He had his critics inside the State Department, who saw him as a political player. Last November, a State Department inspector general report found that in early 2017, Mr. Hook played a role in the reassignment of a nonpartisan career official in his office after conservatives accused her of sympathy to Mr. Obama and the Iran nuclear deal. Mr. Hook denied any political motivation in the reassignment.
His record of diplomatic success was mixed. As Mr. Trump threatened to scrap the Iran deal in 2018, Mr. Hook met regularly with Western European officials in an ultimately futile effort to convince them — especially the Germans — to reopen the accord and toughen its terms. The effort failed, and Mr. Trump’s decision to exit the accord led to a rift with the European powers, Russia and China on Iran policy that remained to this day.
He was among the few top department employees to survive the transition after Mr. Tillerson was ousted and replaced with Mike Pompeo, then the C.I.A. director. Mr. Pompeo gave Mr. Hook the Iran file on a full-time basis, a job he pursued doggedly, including when he personally emailed the captain of an Iranian tanker carrying oil to Syria in violation of U.S. sanctions, offering him millions of dollars if he piloted the ship into the hands of a U.S. ally. (It didn’t work.)