'The Plot Against America' is about what might have happened if the anti-war aviator Charles Lindbergh had stood against Franklin D. Roosevelt in 1940 and won the presidency: the US government negotiates with Nazi Germany and persecutes Jews, including Philip Roth's family. Before Pearl Harbor, there was a conservative isolationist movement in the USA consisting of Americans who didn't see why they should die in wars between European powers. Roosevelt did see why, and he won the 1940 election and pushed America into war.
This novel tries to convince us that 'War is Peace'. Roosevelt, the man who supported Britain's declaration of war on Germany by supplying her with guns, who provoked the Japanese, who used every trick in the book to drag a reluctant America into a massacre, is tarted up as a good guy, whereas Lindbergh, who wanted to keep America out of the war, is an accomplice of Nazi aggression.