Stone House at Manassas Battlefield
The first large-scale invasion of the Confederacy by the Union occurred on May 24, 1861, when Alexandria was seized. It remained in firm control of the Union throughout the Civil War - and between 1863-65, the administration of Abraham Lincoln treated Alexandria as the official capital of Virginia.
Technically, Virginia's secession from the Union became official on May 24, 1861. Early that morning, Union troops crossed the Long Bridge and marched along the extension of the C&O Canal between Aqueduct Bridge and Alexandria. At the same time, Union forces were carried across the Potomac and landed at the Alexandria wharfs. As the Union forces entered Alexandria, the 17th Virginia Regiment retreated westward. No fighting occurred, except when the commander of the Union forces (Elmer Ellsworth) tore down the Confederate flag flying over a hotel at King and Pitt Street. James Jackson, manager of the Marshall House, shot Ellsworth and was killed in return by a Union soldier.1
Seizing Alexandria was a tactical move to protect Washington DC. It eliminated the risk of Confederates placing cannon on the hills of the city and shelling the Capitol or the White House. The Federals proceeded to build a ring of forts around the District, and they deterred most Confederate attacks. In 1864, General Jubal Early swept up the Shenandoah Valley, crossed the Potomac River, and attacked Washington from the north. Abraham Lincoln went to a fort to observe the defense - and Oliver Wendell Holmes is reported to have grabbed him by the arm and shouted "Get down, you fool."
After Virginia joined the Confederacy, the capital was moved from Montgomery, Alabama to Richmond, Virginia. That put the two political capitals just 100 miles from each other. The Confederacy simply needed to survive in order to prove it was an independent nation, but the Union needed to demonstrate it had control over the territory it claimed was still part of the United States. Otherwise, European nations would recognize the Confederacy and provide political, economic, and military support - just as France had provided essential resources for the rebellious colonies to establish their independence in 1776-1783.