The mill itself had nothing to do with war. George Washington Parke Custis, the adopted grandson of George Washington, built it in 1836 on his Arlington estate where the Columbia Turnpike crossed Four Mile Run. By 1861 it was simply a working grist mill, grinding corn and wheat for local farmers, sitting on a country road south of the Long Bridge. Then the war arrived. On May 24, the morning after Virginia voters formally ratified secession, Union regiments poured across the Potomac before dawn and occupied the heights above Alexandria. Within days, picket lines extended out into the countryside. Companies of Michiganders and New Yorkers took up positions at Arlington Mills, sleeping in nearby houses and watching the road south.

Company E of the 1st Michigan Volunteer Infantry was on picket duty that night. Company G of the 11th New York, the famous Fire Zouaves recruited from New York City's volunteer fire companies, was in a nearby house preparing to relieve them. Both regiments were green. The Michiganders had been mustered into federal service exactly one month earlier. The Zouaves had been in uniform less than a month and had recently lost their colonel, Elmer Ellsworth, shot dead while pulling a Confederate flag from a hotel roof in Alexandria. They were jumpy. At about eleven, nine Virginia militiamen, technically still state troops though Virginia had already aligned with the Confederacy, crept up out of the dark and fired a volley at the sentinels. In the confused exchange that followed, one Union soldier was killed and another wounded. One Virginian was wounded. The militiamen melted back into the night.

Historians today struggle to call Arlington Mills a battle at all. It was a skirmish, a picket-line scuffle, the kind of brief contact that happened a thousand times in the war's later years without anyone bothering to record it. But in June 1861 the war was new, and a single rifle volley fired within sight of Washington made national newspapers. The Battle of Fairfax Court House had occurred earlier the same day, a few miles west. Together, the two small actions were among the first organized exchanges of fire between Union and Confederate forces in the eastern theater. They were antecedents, as historian Charles Poland Jr. put it, of the much larger collision at Bull Run on July 21. The country had not yet learned what the war would become.

The mill is gone. The grist mill that George Washington Parke Custis built in 1836 has been replaced by Arlington Mill Community Center, a modern brick building completed in 2013 at 909 South Dinwiddie Street. The Columbia Pike still runs nearby, busy with commuters. Four Mile Run still flows through Arlington toward the Potomac, narrower and more channelized than the stream the millers knew. The 1st Michigan went on to fight at First Bull Run on July 21 and was mustered out three weeks later, its ninety days of service complete. The 11th New York Fire Zouaves never recovered from Ellsworth's death and from the chaos at Bull Run, and the regiment dissolved within a year. The Virginia militiamen who fired the volley are unnamed in the records. They simply disappeared back into the countryside that had sent them.

What makes the site worth remembering is not the fight itself but where it happened. Four Mile Run is now ten minutes from the Pentagon, fifteen from the White House. The land that Custis owned passed through his daughter Mary, whose marriage to Robert E. Lee carried the Arlington estate into one of the war's strangest geographies, the Confederate general's home seized and turned into a Union cemetery while the war was still going. Arlington Mills sat on the southern edge of that estate. It was, in May and June of 1861, the outermost frontier of the United States as the Lincoln administration understood it, a piece of countryside whose loyalty had to be enforced regiment by regiment, picket post by picket post. Nine men with muskets crossing one June night were enough to make the point.