Conversation so far:
What was this during?
During the War of 1812, the British captured the fort in the Siege of Fort Mackinac.
What did this do for him?
The victorious British attempted to protect their prize by building Fort George on the high ground behind Fort Mackinac.
Why was this so terrible?
In 1814, the Americans and British fought a second battle on the north side of the island.
What did this mean to him?
The American second-in-command, Major Andrew Holmes, was killed and the Americans failed to recapture the island.
What did he try and do?
Despite this outcome, the Treaty of Ghent of 1815 forced the British to return the island and surrounding mainland to the U.S.
WHat was so terrible to him?
The United States reoccupied Fort Mackinac, and renamed Fort George as Fort Holmes, after Major Holmes.
Who was helping him?
Fort Mackinac remained under the control of the United States government until 1895 and provided volunteers to defend the Union during the American Civil War.
Did they do during the civil war?
The fort was used as a prison for three Confederate States of America sympathizers.

Question: Who helped him with this?
Self-contained rewrite: Who helped the Unites States with converting the fort to a prison