The Secretary of the Interior is authorized to acquire such lands and interests in land and to enter into such agreements with the owners of land on behalf of themselves, their heirs and assigns with respect to the use thereof as the Secretary finds necessary to preserve, protect and improve the Antietam Battlefield comprising approximately 1,800 acres in the State of Maryland and the property of the United States thereon, to assure the public a full and unimpeded view thereof, and to provide for the maintenance of the site