The American War for Independence (1775–1783) centered the Great Lakes in the middle of international conflict. By the 1770s, as tension between Britain and its American colonies reached a breaking point, Fort Ontario which had been constructed during the French and Indian War (1754-1763), was essentially abandoned.
It was located on Native land as defined by the Royal Proclamation of 1763 and the boundary line established by the subsequent 1768 Treaty of Fort Stanwix.
No American Revolutionary War battles were fought in Oswego, but it was used as a British base of operations and supply. Loyalists used Fort Ontario as a convenient stop on their travels to Canada and as a staging area for small raids and attacks. In 1775, Mohawks sailed from Oswego with British forces to Montreal.
In June 1777, Fort Ontario was the point of assembly of the forces for an expedition under Lieutenant Colonel Barry St. Leger to attack the Mohawk Valley and cooperate with John Burgoyne.
Left un-garrisoned, in 1778 the fort was destroyed by an American detachment under Lieutenant McClelland, who had been sent by Peter Gansevoort at Fort Stanwix.
In July 1779, Continental Army troops from Fort Stanwix set fire to the abandoned Fort Ontario’s parade buildings and wood ramparts to dissuade British use of the site, but the earthworks remained intact.
Throughout 1780–1781, British forces and Loyalist-Iroquois raiding parties intermittently used the remains of Fort Ontario as the starting point for attacks on New York settlements.
In 1780, Sir John Johnson’s force passed through Oswego from Montreal en route for his destructive raid in the Schoharie and Mohawk Valleys.
In 1782, the Governor-General of Canada Major General Frederick Haldimand ordered British troops to permanently garrison at Oswego and build the third Fort Ontario to provide additional protection for Canada.
While the fort’s earthworks were revetted with timbers, horizontal pickets were installed on the outer walls, and blockhouses and a palisade were constructed during the occupation, British troops ultimately surrendered in April 1783.
The Treaty of Paris signed in September 1783 officially ended the Revolutionary War and established the formal border between the United States and British-controlled Canada.
However, Britain retained control of Fort Ontario and six other existing forts at strategic locations along the northern US border for over a decade, until the ratification of John Jay’s Treaty in 1795 reconfirmed the United States’ claim on the Great Lakes posts.
On July 14, 1796, US troops finally relieved the last British garrison at Oswego and raised the American flag over Fort Ontario.
With the border established and restrictions on trade between the United States and Canada in place, US troops were officially withdrawn from Fort Ontario in 1803.
New York militia troops garrisoned at the fort sporadically until the outbreak of the War of 1812.
This essay was drawn from the National Park Service’s 2024 Fort Ontario Special Resources Study, available in it’s entirety with footnotes here.
Illustrations, from above: Map showing the territorial gains of Great Britain and Spain following the French and Indian War with lands held by the British prior to 1763 (in red), land gained by Britain in 1763 (in pink), and lands ceded to the Kingdom of Spain in secret during 1762 (in light yellow); Detail from portrait of Gansevoort by Gilbert Stuart, 1794; and a postcard showing the evacuation of Fort Ontario by the British in 1796.
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