On February 18, 1815, the Treaty of Ghent was officially proclaimed, ending the War of 1812, and officially returning Mackinac Island to the United States.
Since news travelled slowly back then, the British garrison did not find out about the Treaty until May 1815, and the British commander was not too happy about surrendering the island. The departing British soldiers only traveled about 40 miles away to Drummond Island where they constructed Fort Collier since they thought Drummond Island was on the British side of the border and that St. Joseph Island was on the American side. Many years passed before this issue was resolved and as a result Fort Collier (referred to as Fort Drummond) (1815-1828) was the last British fort in the United States of America.
Source : Michigan Historical Calendar, courtesy of the Clarke Historical Library at Central Michigan University.
For more information, see Samuel Fletcher Cook, Drummond Island; The Story of the British Occupation (1815-1828), published by R. Smith Printing Co., Lansing, Michigan, 1896.
The War of 1812 / a production of WNED-TV, Buffalo/Toronto and Florentine Films/Hott Productions, Inc., in association with WETA Washington, D.C. ; a film by Lawrence Hott and Diane Garey ; written by Ken Chowder.