World War 2 and Axis occupation
The USA had entered WW2 in 1944, but it had no chance of winning.
When it surrendered in 1947, the Germans occupied the mainland, while Japan got the rest. The Jews only fared slightly worse than their European cousins, with them being used for slave labour alongside the black communities rather than outright extermination. The Germans were also surprisingly merciful, by helping the Americans rebuild and getting production back to prewar levels.
The Castro Rebellion
The USA finally broke the yoke in 1954, while the Nazis were preparing for war with Japan. Fidel Castro, Che Guevara, the Provost Marshall with an assortment of other rebels stormed capitol hill, citing the Declaration of Independence as a precedent for overthrow. The congress was all arrested for treason. The Castro administration then gave the cover of maintaining the axis pact. By that stage, the Nazi soldiers were already in Russia preparing for the coming war
When the war between Germany and Japan ended, the USA took advantage of the weakened Axis powers' predicament and annexed all of North and South America, as well as cutting off all tribute to Germany and coming public about their military plans.
World War 3
Main Article: World War 3
The USA decided the time was right on July 1959 and began a full-scale assault via the Chukotka Peninsula against the Japanese Empire. The new F-14 and F16 fighters were used to great effect, wreaking havoc on the shattered Japanese military, while the new Abrams tank made mincemeat out of the Japanese Nobunaga tanks.
The USA emerged victorious from the war, with the USA reconstructing the former Axis powers with the help of China and the New Soviet Union. A new conference in Bretton Woods helped found the World Bank and the IMF.
There are 144 States in the Union and it covers all of the Americas and the Philippines. The current US population is 1.4 billion and their economy is one of the strongest in the world (it is currently competing with the PCU and the New Soviet Union for economic power)