The First Battle of Bull Run, called the Battle of First Manassas by Confederate forces, was the first major battle of the American Civil War. The battle was fought on July 21, 1861, in Prince William County, Virginia, just north of what is now the city of Manassas and about thirty miles west-southwest of Washington, D.C. The Union Army was slow in positioning themselves, allowing Confederate reinforcements time to arrive by rail. Each side had about 18,000 poorly trained and poorly led troops. The battle was a Confederate victory and was followed by a disorganized post-battle retreat of the Union forces.

The Confederate States Army, also called the Confederate Army or the Southern Army, was the military land force of the Confederate States of America during the American Civil War (1861–1865), fighting against the United States forces to win the independence of the Southern states and uphold and expand the institution of slavery. On February 28, 1861, the Provisional Confederate Congress established a provisional volunteer army and gave control over military operations and authority for mustering state forces and volunteers to the newly chosen Confederate president, Jefferson Davis. Davis was a graduate of the U.S. Military Academy, and colonel of a volunteer regiment during the Mexican–American War. He had also been a United States senator from Mississippi and U.S. Secretary of War under President Franklin Pierce. On March 1, 1861, on behalf of the Confederate government, Davis assumed control of the military situation at Charleston, South Carolina, where South Carolina state militia besieged Fort Sumter in Charleston harbor, held by a small U.S. Army garrison. By March 1861, the Provisional Confederate Congress expanded the provisional forces and established a more permanent Confederate States Army.