In April 1775, immediately after the battles of
Lexington and
Concord, Parsons, along with colleagues in the Connecticut legislature, began promoting a project to take
Fort Ticonderoga from the British, securing commitments of both public and private funds to underwrite the expedition.
Like most active politicians of the period, Parsons served as a militia leader. He was appointed Major of the 14th Connecticut, Militia Regiment in 1770. In 1775, he was commissioned Colonel of the
6th Connecticut Regiment, a new regiment raised "for the special defence and safety of the Colony". In June he was ordered to lead his regiment to
Boston, where he fought in the
Battle of Bunker Hill. He remained in Boston until the British evacuated the city in March 1776.
In August 1776 Congress appointed Parsons Brigadier General in the
Continental Army. He was ordered to
New York with his brigade of about 2,500 men. Stationed in
Brooklyn, Parsons was in the thick of the fighting with British troops under
Lord Sterling at
Battle Hill on
August 17 1776. He took part in the Council of War on
August 29, at which it was decided to retreat from New York. Parsons successfully transported his men from
Long Island, joining the main body of the army as it withdrew from the city.
While in New York, Parsons played a central role in the American efforts to destroy the British fleet.
David Bushnell, an inventor from Connecticut, had devised a
submarine which he planned to use to place
torpedoes on British ships. Parsons selected his brother-in-law, Captain Ezra Lee, to undertake this risky mission. Lee succeeded in reaching the British flagship Asia undetected, but was unable to attach the torpedo to its hull. The bomb exploded, much to the consternation of the British, but without causing any harm to the ship.
After the retreat from New York, Parsons' brigade was assigned to General
Rufus Putnam's division north of the city. He fought in the
battle of White Plains. In January 1777, he returned to Connecticut to help recruit the
Connecticut Line to bolster depleted Continental forces. He led raids on
Tory enclaves on Long Island, and took part in efforts to defend Connecticut towns against raids by British forces under General
William Tryon.
In the winter of 1777–78, Parsons took command of
West Point, and began rebuilding its fortifications. At the end of 1778, he joined Connecticut troops at winter quarters in
Redding. In December 1779, Parsons took command of
Putnam's Division, and spent the following months recruiting, training, and trying to engage British General
George Clinton in battle. The high point of this period was the discovery, in September 1780, of
Benedict Arnold's treacherous scheme to surrender West Point to the British. Parsons served on the board of officers which tried Arnold's accomplice, Major
John André, and ultimately sentenced him to death.
On October 23, 1780 Parsons was promoted to Major General. In the winter of 1781 he helped suppress the mutinies of soldiers in
Pennsylvania and
New Jersey, and took part in efforts to clear out Tory militias in
Westchester, north of New York. After months of containing the British troops in New York, American forces, now bolstered by
French reinforcements, departed for
Virginia. Parsons and his troops were left behind to keep the British contained.
In July 1782, following the British surrender at
Yorktown, Virginia, Parsons – broken physically and financially – tendered his resignation to Congress. Forty-five years old, he had served continuously since the
Lexington Alarm of 1775.