English: Looking east at the Civil War Unknowns Monument, located in Section 26 on the grounds of Arlington House (the Robert E. Lee Memorial) at Arlington National Cemetery in Arlington, Virginia, in the United States.

Robert E. Lee vacated Arlington House on on April 20, 1861. Mary Custis Lee vacated the property on May 14, and the armed forces of the United States occupied the house and grounds on May 24. On January 11, 1864, the estate was formally seized by the U.S. government for nonpayment of taxes. The first military burial on the grounds occurred on May 13, 1864. Secretary of War Edwin M. Stanton approved the establishment of a military cemetery on the grounds on June 15, 1864. Quartermaster General of the Army Montgomery C. Meigs was a native Georgian, but loyal to the United States. Once a close friend of Lee's, Meigs considered him a traitor for joining the armed forces of the Confederate States of America. Determined to desecrate the grounds of Lee's estate and render it uninhabitable, Meigs began burying large numbers of war dead on the grounds of the house and throughout the estate.

Arlington House sits on a ridge that is in the form of a reverse capital letter "L", with the short leg running north-south and the long leg running northeast-southwest. Mrs. Lee's rose garden was at the intersection of these two legs.

In 1865, Meigs decided to build a monument to Civil War dead in the center of this rose garden. His goal was to desecrate the rose garden, and make it politically impossible for the Lees to remove the mass grave. U.S. Army troops were dispatched to investigate every battlefield within a 35 mile radius from the city of Washington, D.C. The bodies of 2,111 Union and Confederate dead were collected, most of them from the battlefields of First and Second Bull Run as well as the Union army's retreat alone the Rappahanock River. Some of these had been interred on the battlefield, but most were full or partial remains discovered on the field of battle. None were identifiable.

U.S. Army engineers dug a circular pit about 20 feet wide and 10 feet deep was dug into the earth. The walls and floor were lined with brick, and it was segmented it into compartments with mortared brick walls. Into each compartment were placed a different body part: skulls, legs, arms, ribs, etc. The vault was half full by the time it was ready for sealing in September 1866. The vault was sealed with concrete and soil. Meigs designed a a 10-foot-tall grey granite and concrete sarcophagus modeled after the Ark of the Covenant described in the book of Exodus in the Jewish and Christian Bible.

The memorial is a rectangular sarcophagus about 12 feet long and 4 feet wide. It stands on a slightly larger base of rough-hewn dark grey granite blocks mortared together. This base is three stones high, or about three feet. A second base of large light grey granite slabs about 1 foot high sits on top of the first base. A third base, consisting of a single light grey granite slab, sits on the second base. This third base is slightly smaller than the first two, but slightly larger than the memorial itself. The sarcophagus itself consists of two long light grey granite slabs, with the shorter ends formed by sandwiching a smaller slab between these two. On the west-facing side is a dedicatory inscription. Four light grey granite sections about six to nine inches high sit atop the sides, decorated with pilasters. Like the walls of the sarcophagus, these consist of two long, unbroken sections with smaller sections inserted between them at the ends. These sections are mortised, so that all four sections appear to meet at the corners. A slightly larger light grey granite base flares out above the pilaster section. Unlike the lower section, longer sections are sandwiched between the end sections. (This enables friction to help tie the monument together.) The cover of the sarcophagus consists of a single large light grey granite slab. Twenty-two bas-relief stars inside circles exist above the lip of the lid. Four stylized, partial fleur-de-lis face outward at each corner. The top of the lid is rounded. The entire monument is about 12 feet high at the uppermost portion of the lid.