On September 5, 1814, Flagg became a militia lieutenant in a small scouting detachment of teenage boys from the Plattsburgh Academy raised by captain Martin James Aikin (1791-1828).

[2] General Alexander Macomb praised the Aiken's Volunteer Rifle Company for not falling back in disorder with the bulk of Mooers' militia during the first encounters with the British invading force in the Battle of Plattsburgh.

[3][4] Following retreat, Aiken's volunteers manned the bank of the Saranac River to prevent the enemy from crossing it; one boy was killed in the resulting skirmish.

In 1826, Congress awarded each of them, including Flagg, "one rifle, promised them by General Macomb, while commanding the Champlain Department, for their gallantry and patriotic services as a volunteer corps during the siege of Plattsburgh.

[8] He was one of the leading members of the Albany Regency, who exercised a great deal of control over New York's Republican Party, along with Van Buren, Silas Wright, and William L.

[9] In a curious encounter, Flagg was introduced to Alexis de Tocqueville in Albany in 1831, and invited him to walk during the 4th of July parade with the state dignitaries.