Rolland is correct. Although the Treaty of Paris of 1783 said that Britain would evacuate all posts w/in the new United States, they did not. Scattered posts from present-day Vermont to present-day Michigan remained in British hands until Jay's Treaty of 1795. Niagara was one of these British held forts on U. S. soil.
 
Not all points in the 1783 Treaty were honored. We were slow in allowing British debts to be collected & never did return confiscated British real estate to its pre-war owners. Neither did the British ever return captured & run-away slaves to their pre-war masters.
 
David
 
David Roberts
Hollywood, MD
 
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From: [log in to unmask] href="mailto:[log in to unmask]">NWDB2000
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Sent: Monday, November 27, 2006 12:57 PM
Subject: Re: [NYHIST-L] Tomorrow is the 223rd Anniversary of Evacuation Day

"Britain gained control of Fort Niagara in 1759, during the French & Indian War, after a nineteen-day siege. The British held the post throughout the American Revolution but were forced, by treaty, to yield it to the United States in 1796. "  https://oldfortniagara.org/history/
 
Would seem to exceed the Evacuation of New York date quoted below:
 
"On November 25, 1783, the British Army boarded their naval vessels and
Evacuated New York City (then only coterminous with New York County), their wartime
Headquarters and their last military position in the United States during the
Revolutionary War."
 
Rolland Miner
Fogg of War, Book II - Assault on Carillon 1758
An Historical Novel of the F&I War