Until now, I have generally been reluctant to label Donald Trump the worst president in U.S. history. As a historian, I know how important it is to allow the passage of time to gain a sense of perspective. Some presidents who seemed awful to contemporaries (Harry S. Truman) or simply lackluster (Dwight D. Eisenhower, George H.W. Bush) look much better in retrospect. Others, such as Thomas Jefferson and Woodrow Wilson, don’t look as good as they once did. So I have written, as I did on March 12, that Trump is the worst president in modern times — not of...