The United States Army is the land component of the United States Military and serves to protect the citizens & government of the United States from either external or internal harm, and can be deployed abroad in a variety of methods and functions. It is highly regarded as one of the best armies on Earth at many times throughout human history. The Army appears in almost every King Kong film due to the franchise's in-universe heavy involvement of the United States and American citizens.
In the 2005 film, set in 1933 (the era of the original King Kong film) a pre-World War II US Army (likely the National Guard as those are units that are authorized to be deployed in American cities without needing the approval of the President of the United States) acted as minor antagonists to the story, responding to Kong's rampage in New York City with infantry, self-propelled artillery, armed trucks and light vehicles in an attempt to kill the giant ape, but which puts them into conflict with the film's protagonists. As for the latest film, released in 2017, Kong: Skull Island, now set in 1973, many of the characters onscreen are members of the "Sky Devils," a US Army helicopter unit that lost much of its aircraft and men fighting against both Kong and other vicious native creatures of Skull Island.