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, who is mentioned offhand in the chapter ending descriptions in particular at the end of Episode 3. With references to Blazkowicz being a hero at the end of the war, when he returns to America.
It's also introduced in the official guides that B.J. Blazkowicz's country of origin is the United States and that his family lives 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 (or late as 1947) (when the story takes place), the U.S is working on the atomic bombs during the Manhattan Project and has plans to soon use them to end the war. However, the Normandy beach landings have not yet taken place.
See American Territories.