Like in our timeline, the United States of America was a thriving nation with a stable government until the Stock Market Crash of 1929, where the United States entered the Great Depression, expereincing high unemployment and a declining economy. The history of the United States diverged from our own in 1933, when President Franklin Delano Roosevelt was assassinated. With Roosevelt not having a chance to enact the New Deal, the Great Depression persisted in the US well into 1940s along with its policy of isolationism. As a result, the United States was both unwilling and unable to assist the Allies against German aggression when World War II broke out in 1939. 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. 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 destroye 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).