[Quoted from Wikipedia, John Armstrong, Jr.]
After early education in Carlisle, John Jr. studied at the College of New Jersey (now Princeton University). He broke off his studies in Princeton in 1775 to return to Pennsylvania and join the fight in the Revolutionary War. His service record is sometimes confused with several other John Armstrongs in the war, including his father.
The young Armstrong initially joined a Pennsylvania militia regiment and the following year he was appointed as aide-de-camp to General Hugh Mercer of the Continental Army. In this role, he carried the wounded and dying General Mercer from the field at the Battle of Princeton. After the general died on January 12, 1777, Armstrong became an aide to General Horatio Gates. He stayed with Gates through the Battle of Saratoga then resigned due to problems with his health. In 1782 Gates asked him to return. Armstrong joined General Gates' staff as an aide with the rank of major, which he held through the rest of the war.