The United States is a major country during World War II consisting of 48 states. Its main military force is the U.S Army.
In the original game, the United States are mentioned in relation to the President Franklin D. Roosevelt and/or Truman who is mentioned offhand in the chapter ending descriptions in particular at the end of Episode 3.
It's also introduced in the official guides that B.J. Blazkowicz's country of origin is the United States, and its implied to be in Milwaukee, Wisconsin (while his parents were immigrants from Poland, he himself was born in the States, and thus a natural-born citizen). In addition its made a point that he is a United States soldier in the documentation, and various Nazi opponents make reference to this fact by calling him the American.
Spear of DestinyEdit
Franklin D. Roosevelt appears in the ending for the game and its two expansion packs within the White House in Washington D.C. to thank B.J. Blazkowicz for his successes at stopping the Nazis from using Spear of Destiny for evil purposes. The backstory for Mission Pack 2: Return to Danger tells of Hans von Schlieffen ("The Axe") and his troops attempt to invade the United States to steal back the Spear of Destiny, and take it to a secret Scandinavian Base. He was the only survivor.
By 1945 (when the story takes place), the U.S has had successful tests of atomic bombs during the Manhattan Project and has plans to soon use them to end the war.
The New OrderEdit
The United States is among one of the last few countries to fall to the German might.They surrender in 1948 after the destruction of Manhattan by an atomic bomb. The President announced it's surrender during a speech a day later. With Hitler giving a speech to his American Nazi supporters and Horton Ho 229 Jets flying overhead.
Soon after the Germans did a victory tour through the streets of the East Coast, and ordered soldiers to stand down and be issued work cards, to be redistributed into new labor. It's also known that they controlled New York, and most of the east coast up to 1960, and planned to rebuild Manhattan by 1965.
J states that the average white American "patriot" (including B.J.) would actually have fit-in with The New Order, and agreed with the Nazis, and most became enforcers for them after the U.S fell ("before the war you were the Nazis"). J, also mentions that the "The Man" (his name for the US government), "ran scared...mother-fuckin gave up...Nazi asked the Man to get down on his knees, and the Man was all too happy to pucker up, and start kissing Nazi ass, like it was the most natural thing in the world". That the Man started throwing its 'degenerates'/'undesirables' into the gas chambers, just like the Germans.
The game is vague as to the western states, one newspaper (no date given) suggests that resistance made it very hard for Germans to keep control of most of the United States. However, the most of the United States continued to be a challenge for the Germans, as most people took up arms and continued to resist the Germans, despite the Germans attempts to kill innocent civilians in retribution. There is no specific explanation as to the outcome of the resistance, but it may still be active because Nazi forces are spread wide and thin around the world to keep order. This is evidenced by the Kreisau Circle, which is active, even though it is in the heart of Nazi territory.
There are hints that perhaps as far west as Texas may still be relatively free. But these maybe areas where Germans may still have a frontline, although the main frontline is still in Africa by 1960. By the end of the game, it could be assumed due to the destruction of Deathshead and his compound, which dealt a devastating blow to the Nazi's war efforts and slowed down the war in Africa, the Nazi's grip on the world, including the United States, is slowly crumbling.
The New Colossus Edit
Initial trailers for Wolfenstein II show that the Germans have a strong hold on the City of New Orleans, suggesting they control America's East Coast up to and beyond the Mississippi River.