American Civil War: Union troops occupy Alexandria, Virginia, with Colonel Elmer E. Ellsworth becoming the first Union officer to be killed during the war.

The American Civil War was a civil war in the United States between the Union and the Confederacy, which was formed in 1861 by states that had seceded from the Union. The central conflict leading to war was a dispute over whether slavery should be permitted to expand into the western territories, leading to more slave states, or be prohibited from doing so, which many believed would place slavery on a course of ultimate extinction.

The Union was the central government of the United States during the American Civil War. Its civilian and military forces resisted the Confederacy's attempt to secede following the election of Abraham Lincoln as president of the United States. Lincoln's administration asserted the permanency of the federal government and the continuity of the United States Constitution.

Alexandria is an independent city in Northern Virginia, United States. It lies on the western bank of the Potomac River approximately 7 miles (11 km) south of D.C. The city's population of 159,467 at the 2020 census made it the sixth-most populous city in Virginia and 169th-most populous city in the U.S. Alexandria is a principal city of the Washington metropolitan area, which is part of the larger Washington–Baltimore combined statistical area.

Elmer Ephraim Ellsworth was a United States Army officer, close personal friend of the 16th President of the United States Abraham Lincoln and, law clerk who was the first conspicuous casualty and the first Union officer to die in the American Civil War. He was killed while removing a Confederate flag from the roof of the Marshall House inn in Alexandria, Virginia.