SOUTHAMPTON, ONT.—Wearing blue rubber gloves, Ken Cassavoy is carefully unfolding a threadbare flag on a boardroom table at the Bruce County Museum & Cultural Centre.

Though greatly faded, the red, white and blue of a British Red Ensign are clearly visible — a Union Jack in the top left-hand corner, surrounded by a sea of red.

This is the first time Cassavoy has unpacked the flag since he fetched it home on loan from Annapolis, Md., where for two centuries it has been a war trophy at the U.S. Naval Academy Museum.

As flags go, the ensign isn’t shy. It’s nearly 8 feet tall and is still almost 10 feet long, even after being shortened by about 4 feet when, at some point, the naval museum put a linen backing on it....