Civil War Defenses of Washington National Historical Park Act - Redesignates the Civil War defenses of Washington, which were constructed at the beginning of the Civil War in 1861 as a ring of fortifications in the District of Columbia, Maryland, and northern Virginia to protect the nation's Capital, as the Civil War Defenses of Washington National Historical Park.

Includes in the Park all areas associated with the Civil War defenses of Washington that are currently owned by the federal government and under the administration of the National Park Service (NPS), including Battleground National Cemetery.

Allows certain eligible sites and certain additional sites associated with the Civil War defenses of Washington to become affiliated with the Park pursuant to a cooperative agreement under this Act.

Directs the Secretary of the Interior to study and consider creative and cost-effective ways to facilitate how the storied history of the Civil War for both the North and the South, including the history of the defenses of Washington and the Shenandoah Valley Campaign of 1864, can be assembled, arrayed, and conveyed for the benefit of the public.