Go with me on this one - let's say that US Senator Robert Taft, a strong conservative and noted isolationist, had secured the Republican presidential nomination in 1940, then went on to narrowly defeat FDR in the general election. However, despite his isolationism, let's say the situation with Japan still deteriorates to the point where Pearl Harbor, or something like it, occurs, and Germany still declares war upon the United States.
How would American involvement in World War II be different, given that Taft was politically the polar opposite of FDR? What strategy would he insist upon the US military taking, and how would his relationship with the other Allied powers have been different than FDR's (beyond, I assume, greater skepticism toward Stalin)? What sort of postwar settlement would he have insisted upon for Europe and the rest of the world? How would he have navigated postwar geopolitics, however they would have ended up looking here?