The United States Army, the largest branch of the United States Armed Forces, was formed on June 14, 1775. The mounted reconnaissance, surveillance, and target acquisition branch of the U.S. Army was known as the U.S. Cavalry.

From 1861 to 1865, the army of the federal government of the United States of America was known as the Union Army, while they battled eleven states that seceded from the Union, known as the Confederate States of America.

One of the battles of the American Civil War was known as the Battle of Chattanooga, where the Confederate forces, led by General Beauregard Tannen, went into battle against the Union forces, led by Ulysses S. Clayton. After Jules Brown, Verne Brown, and Marty McFly intervened, brothers, cousins, and others on both sides of the war reconciled, forming a truce, changing history so that the battle had never occurred.

On September 2, 1885, the cavalry began a chase of Pohatchee Indians, which ran into the DeLorean time machine. They chased the Pohatchee past the bear cave and continued after them as Marty McFly watched from the safety of the cave.

The U.S. Army participated in World War I as a member of the Allied Forces between 1914 and 1918. Arthur McFly enlisted in the U.S. Army in World War I and was sent to France, however he never fired a single shot. The military brass discharged him when they discovered he had lied about his age, and was actually only sixteen.

In 1985, in the ABC timeline where Biff Tannen owned Hill Valley, the corrupt Hill Valley Police Department had somehow gotten their hands on a U.S. Army tank, and were using it to keep order in the crime-infested Courthouse Square.

The Manhattan Project, the nuclear weapons program of the United States, was overseen under the auspices of the United States Army from 1942 to 1946. It was led by Major General Leslie Groves of the U.S. Army Corps of Engineers. It developed the nuclear bombs that the United States dropped on Japan during World War II.

One of the scientists hired to work on the project was Doc Brown, who was hired in 1943, during his tenure as a professor at the California Institute of Technology. Doc had managed to convince Robert Millikan, the president of Caltech, to arrange an interview with Groves and the Manhattan Project's director, Vannevar Bush. Despite a disastrous interview, the physicist J. Robert Oppenheimer, who was the the director of the Los Alamos National Laboratory that designed the actual bombs, awarded him the position.

On September 27, 1944, a civil defense warden catches Verne Brown and Marty McFly trying to break into Doc Brown's apartment, and he impounds the DeLorean time machine.

Without a way home, Verne and Marty spend the night at a victory garden in Courthouse Square. Marty is attracted to a woman named Rosie, earning him the ire of Sargent Frank Tannen. While running away from, Tannen, Marty accidentally enlists in the U.S. Army. Meanwhile, Verne befriends Rosie's sister, and soon discovers that when she gets older, she becomes his dancing instructor in 1992.

Eventually, Marty manages to escape Frank Tannen when Verne and Dorothy enter and win a dancing competition at a USO show. Marty and Verne are able to return home to 1991, and Verne resumes dancing lessons with Rosie's sister, although he is now enthusiastic about dancing.

In 1962, the U.S. Army, under the auspices of the Department of Defense, sought to invent time travel in order to ensure that the Cuban Missile Crisis never occurred. To accomplish that, Groves and Colonel Lomax came to Emmett Brown, as he had left a lasting impression on Groves while working on the Manhattan Project. After having regrets about accepting their offer, Doc used his prototype temporal field capacitor to send a message to himself in the past. This caused an electrical fire, which burned down the Brown family mansion. As Lomax and Groves believed him to be an insurance thief, the fire prevented Groves from ever asking Emmett about time travel.

After the Cuban Missile Crisis had ended, Lomax and Groves then went to another scientist, Marcus Irving, letting him believe that he was their first choice for the project. Irving agreed to the project, and spent 24 years working on time travel. He could never solve the problem of excess flux energy, so his time experiments were never a complete success. Finally, in January 1986, government funding for Irving's project was terminated.

Irving then sought to find out everything he could about the project, and discovered that he was not the first choice. He was jealous of Emmett Brown, for turning them down and for causing him to waste 24 years of his life. While tracking down Doc Brown, Irving discovered that he had built a working DeLorean time machine. He sabotaged the DeLorean and used Doc's design to build a flux capacitor, enabling him to travel through time as well.


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