On May 23, 1861, Virginia citizens ratified the convention's secession ordinance by a three-to-one margin. At dawn the next day, Union troops crossed the Potomac. Two columns moved south from the bridges. To the north, Colonel Orlando B. Wilcox's 1st Michigan Regiment landed on what is now the grounds of Arlington National Cemetery, occupied the Custis-Lee House - the prewar home of Robert E. Lee, who had resigned his U.S. Army commission a month earlier - and pushed south up the ridge. To the south, Colonel Elmer E. Ellsworth led the 11th New York Fire Zouaves into Alexandria. Ellsworth was a Lincoln family friend, a personal favorite of the President. He spotted a Confederate flag flying over the Marshall House hotel, climbed the stairs to remove it, and was shot dead by the proprietor James W. Jackson on the way down. He became one of the first Union officers killed in the war. By the end of the day, Union engineers were already laying out earthworks on the ridge above Wilcox's position. Fort Albany and Fort Scott went up within weeks - the southern anchor of what came to be called the Arlington Line. The ridge stayed under Union control until the end of the war.

The Hume School, built in 1891, is the oldest school building in Arlington County. It taught local children continuously until 1958 and now houses the Arlington Historical Society and Museum at 1805 South Arlington Ridge Road. The school is on the National Register of Historic Places. A short walk away, at 1301 South Arlington Ridge Road, stood the Little Tea House - opened in 1920 by Gertrude Crocker, a women's rights activist who wanted to support herself with a business of her own. The Tea House became a small Washington-area institution for forty years. Eleanor Roosevelt ate there. Amelia Earhart ate there. Oliver Wendell Holmes Jr. - the Civil War veteran turned Supreme Court justice who had been wounded as a young first lieutenant at Ball's Bluff in 1861, just up the river - ate there. It was also one of the first places in segregated Arlington where racially mixed groups could meet for a meal. Crocker leased and eventually sold the business to Gertrude Allison, after whom it was renamed Allison's Little Tea House. It was demolished in 1963 to make room for a high-rise apartment building. The only surviving fragment is a small stone tower at the corner of South Lynn Street - now used as a pool maintenance shed - that once belonged to the tea house grounds. There is no marker.

Prospect Hill, the northern tip of the ridge, is a small Arlington County historic site at the corner of South Arlington Ridge Road and South Joyce Street. A brick mansion built in 1841 by James Roach - a contractor whose company had supplied the brick and stone for the Aqueduct Bridge, the Alexandria Canal, and the Alexandria, Loudoun and Hampshire Railroad - once stood here. The Union army wrecked the house and grounds in 1861 to build Fort Albany. The mansion limped along through the rest of the nineteenth and twentieth centuries until it was demolished in 1965 to widen Shirley Highway. The site itself remained. After September 11, 2001, when American Airlines Flight 77 struck the Pentagon a mile and a half east-northeast of Prospect Hill, the elevation gave photographers and television crews a clear sight line to the damaged west face of the building. The images that came from this hill in the following hours and days became some of the most reproduced photographs of the attack. A small Pentagon attack memorial was placed at the overlook later. The hill is also the spot where, before the Pentagon existed, Continental Army troops under George Washington and French troops under Rochambeau marched south on Old Georgetown Road in September 1781 on their way to Yorktown. The road they used is now Army Navy Drive.

On May 23 and 24, 1865, the Union Army held the Grand Review of the Armies down Pennsylvania Avenue - 150,000 soldiers marching in formation through Washington in two days, the final ceremonial parade of a war that had just ended at Appomattox. After the parade, the regiments crossed the bridges to Arlington Ridge and bivouacked there. For many Union soldiers, the ridge was their final encampment in uniform. They mustered out from the bluff that they had taken on May 24, 1861 - the same day, four years apart. Arlington Ridge Road, once a major thoroughfare running from Glebe Road up to Rosslyn, was truncated in 1942 by the construction of the Pentagon and Shirley Highway (now Interstate 395). It was reduced from four lanes to two in the 1980s. The neighborhood that grew up along it became a battleground in 1968 over the parking problem - Crystal City commuters were swarming the residential streets to avoid downtown parking fees - and the Arlington Ridge and Aurora Hills civic associations sued the county. In 1977 the United States Supreme Court ruled in Arlington County Board v. Richards that residential parking permits did not violate equal protection. Communities across the country use the precedent today. The ridge has spent 165 years being smaller than people thought it was - and bigger than people expected.