In 1863, during the American Civil War, Fort Machias was built adjacent to the ruins of Fort O'Brien. It was a five-gun earthworks fort with Napoleon 12-pound cannon. It was actively manned from 1863 to 1865. In 1923, the U.S. government transferred the sites of both forts to the state of Maine. As of 2006, they are managed as a historic site by the Maine Bureau of Parks and Lands. In 1969, they were added to the National Register of Historic Places, as structure #69000024.