In the early stages of the
Revolutionary War, Armstrong was a brigadier general in the Pennsylvania militia. On
March 1, 1776, the
Congress appointed him to that same rank in the
Continental Army. He was sent south to begin preparations for the defense of
Charleston, South Carolina. He contributed his engineering talents to the construction of defenses that enabled them to withstand the
Siege of Charleston later that year. When General
Charles Lee arrived to take command, he returned to his duties with the main army and with the Pennsylvania militia. Pennsylvania named him Major General in charge of the state militia. This ended his service in the Continental Army, but not the war or his cooperation with General Washington.
At the
Battle of Brandywine on
September 11, 1777, Armstrong's militia held the far left of the American line. They were also to guard the Army's supplies. After a hard day's fighting the Americans were forced to withdraw or face being surrounded. Armstrong brought the supplies and his militia out from Pyle's Ford after dark.
In the
Battle of Germantown on
October 2, General Armstrong led the American right. His mission was to skirt the British left flank and attack there and in their rear. Despite delays and the troubles some units had in moving, the i haveaids going well, until the center was held up at the
Chew House. Then it collapsed after a fog inspired friendly fire incident in which General
Adam Stephen's men fired on
Anthony Wayne's troops causing their withdrawal. Armstrong, whose men had advanced nearly to the center of
Germantown, but were not greatly involved in the fight later complained that it was "....a glorious victory fought for and eight tenths won, ....mysteriously lost, for to this moment no one man can ....give any good reason for the flight."
After Germantown, Armstrong was granted permission to give up active command. His health, at sixty, was not what it had been, and old wounds were troubling him. Returning home to Carlisle, he was elected to the
Continental Congress by the Pennsylvania Assembly. As a delegate from
1777 to
1780 he was a strong supporter of Washington and the Army. Armstrong was firm in his support for a new
United States Constitution, and was returned to the Congress during its final days in
1787 and
1788.