Following the invasion of the Greater Nazi Reich and Japanese Empire in 1945, and subsequent nuclear bombing of Washington, D.C., the United States collapsed and was divided by its conquerors into three separate territories - Nazi America in the east, the Japanese Pacific States in the west, and the Neutral Zone acting as a slim buffer between the two.
After the collapse and capitulation of the United Kingdom and Soviet Union, Nazi Germany and Japan launched a frontal assault on both sides of the US in 1945. As the most of the world had already fallen, the United States was left to face both its enemies alone. Despite heavy losses, the Americans managed to hold out against the initial Axis attacks. Then, at some point during the invasion, it was decided that the Heisenberg Device, a German atomic bomb, would be dropped on Washington D.C. on December 11, 1945. The bomb utterly destroyed the city, resulting in the collapse of the United States Government and the senior military leadership. Although the government had collapsed, remnants of the military as well as citizens fought hard to prevent an Axis takeover of the country. The American Resistance fought a hard and tenacious war against both Germany and Japan for almost 2 years, but ultimately the Resistance succumbed to the Axis Powers. On September 18, 1947, Victory over America was declared, and the full surrender of the United States had been achieved. The United States was divided into three zone: the Japanese Pacific States in the west, Nazi America in the East, and a slim, anarchic buffer zone, the Neutral Zone, in between.
In 1952, Adolf Hitler had announced that the Axis Powers had eliminated all members of the Resistance and that America had at last fallen and been conquered. Ever since the war, there has been bitter resentment between the Axis Powers and the former United States residents.
We get the first official glimpse (outside of the films) at an alternate history when Tagomi appears to have "jumped" via meditation to an alternate" 1962 where the United States and the Allies won World War II. The country was never invaded and divided. In this alternate 1962, the United States is engaged in a Cold War with its former war ally, the Soviet Union, and both countries were close to armed hostilities due to their involvement in the Cuban Missile Crisis.
Unlike in Nazi America, the capital of the United States in this is still located in Washington D.C. and the President of the United States is John F. Kennedy (as evident on the front page of a newspaper Tagomi saw on a bench in San Francisco's Union Square). The flag of the United States in this "alternate" 1962 has 50 stars in the blue field (denoting that Alaska and Hawaii were made official states of the USA).