The Divider: Trump in the White House, 2017-2021 by Peter Baker and Susan Glasser:
Trump’s New York friends knew what Washington would find out: he planned to live in his own reality in the White House just as he had in Trump Tower. The uncomfortable truth for those encountering him for the first time—including much of his own staff—was that Trump really was what he seemed to be, and he had come to office without a plan for the four-year term that neither he nor his campaign had expected to win. It was an oft-cited fact that Trump was the only president never to have served a single day in either government or military service before being elected. If anything, that understated how unprepared he was for the business of governing. He was probably the least knowledgeable new president in the modern era.
He did not know that Puerto Rico was part of the United States, did not know whether Colombia was in North America or South America, thought Finland was part of Russia, and mixed up the Baltics with the Balkans. He got confused about how World War I started, did not understand the basics of America’s vast nuclear arsenal, did not grasp the concept of constitutional separation of powers, did not understand how courts worked. “How do I declare war?” he asked at one point, to the alarm of his staff, who realized he was unaware that the Constitution prescribes that role for Congress. He seemed genuinely surprised to learn that Abraham Lincoln had been a member of the Republican Party. “He knew nothing about most things,” observed one top aide. Advisers soon realized they had to tutor him on the basics of how government worked.