Fort Ticonderoga, one of the nation’s oldest and most significant historic sites, is so financially strapped that its trustees are considering selling off some of the fort’s vast collection of artifacts, including artwork believed to be worth millions.

Before the Civil War and Antietam, the bloodiest battle fought on American soil was here, on a narrow but strategically vital strip of land between Lake Champlain and Lake George.A 15,000-strong British army sailed up Lake George 250 years ago this...