The National Park Service is commemorating a key moment in the Civil War that marked the Union’s defense of Washington.

The commemoration is planned for Fort Marcy Park in McLean on Saturday. In September 1861, Union Troops marked across Chain Bridge near the site to claim a key point in defending the nation’s capital.

Fort Marcy is one of 68 forts built around the city during the Civil War. Two forts protected the Chain Bridge crossing over the Potomac River. It was one of the few bridges into Washington from Virginia at the time. Fort Marcy is one of only a few such forts that survive 150 years later.

Reservations are recommended for some ranger-led hikes and tours. Saturday’s commemoration runs from 11 a.m. to 3 p.m.