[9][10] In 1758, Colonel Armstrong led 2,700 Pennsylvania provincial troops on the Forbes expedition, the approach of which compelled the French to vacate and blow up Fort Duquesne.

"[12] In the early stages of the American Revolutionary War, Armstrong was a brigadier general in the Pennsylvania militia.

He contributed his engineering talents to the construction of defenses that enabled them to withstand the Battle of Sullivan's Island 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.

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.

[5] After his service in the War ended, Armstrong returned home to Carlisle, where he was elected to the Continental Congress by the Pennsylvania Assembly.

[8] Armstrong was firm in his support for a new United States Constitution, and was returned to the Congress of the Confederation during its final days in 1787 and 1788.

One of these, the Carlisle school board, led him to originally oppose Dr. Benjamin Rush's proposal to start a college in the town.