The US Army was a branch of the military of the United States of America.
In 1958, during the Cold War, Colonel Stark of the US Army made a deal with the Viperox Lord Azlok to destroy the Soviet Union. Stark was convinced by the Tenth Doctor to turn against the Viperox. (TV: Dreamland)
In 2017, the US army was at a standstill with the Chinese army and the Russian army when in a war zone, Turmezistan, a 5000 year old pyramid appears overnight. (TV: The Pyramid at the End of the World)