Both North and South scramble to create armies after the fall of Fort Sumter in April 1861.

Lincoln declares a state of “insurrection” on April 15 and calls for seventy-five thousand volunteers to serve for three months to confront in the South, "combinations too powerful to be suppressed in the ordinary way".

This immediately causes the secession of four additional states, including Virginia.

The first major battle of the war ends in a Confederate victory in the First Battle of Bull Run at Manassas Junction, Virginia, on  July 22, 1861.

Over twenty-eight thousand poorly trained Union forces under Brigadier General Irvin McDowell and Major General Robert Patterson are routed by some thirty-two thousand Confederates commanded by Brigadier General Pierre Gustave Toutant Beauregard and Major General Joseph E. Johnston, as well as Colonel Thomas Jonathan Jackson (later to be known as Stonewall Jackson).

The Union has the upper hand at first, nearly pushing confederate forces holding a defensive position into a rout, but Confederate reinforcements under Joseph E. Johnston arrive from the Shenandoah Valley by railroad, and the course of the battle quickly changes.

A brigade of Virginians under the relatively unknown brigadier general from the Virginia Military Institute, Thomas J. Jackson, stands its ground, which results in Jackson receiving his famous nickname, "Stonewall".

The Union forces stampede back to Washington in panic, losing four hundred and eighteen of their number to the Confederates' thirty-eight.

The Confederates also list 1,522 wounded and 12 missing.

Union wounded number 1,011, and 1,216 are reported missing.

The shock of defeat at Bull Run galvanizes the Union, which calls for five hundred thousand additional recruits.

The defeated General McDowell is replaced on July 24 by Major General George B. McClellan as commander of the Union's Division of the Potomac; McClellan is tasked with forging the army into a competent fighting force.

Union forces capture Forts Clark and ...

...Hatteras on the North Carolina coast by August 29.

Four slave states—Delaware, ...

...Missouri, ...

...Maryland, and ...

...Kentucky—after prolonged contests in their legislatures, had voted by September 11 to remain in the Union.

On September 18, the Kentucky Legislature takes steps to oust Confederate forces.

Union forces under Colonel Edward Baker are defeated by Confederate troops at Ball's Bluff in Loudoun County, Virginia, on October 21, 1861, in the second major battle of the war.

Baker, a close friend of Abraham Lincoln, is also killed in the fighting.

The operation had been planned as a minor reconnaissance across the Potomac to establish whether the Confederates were occupying the strategically important position of Leesburg.

A false report of an unguarded Confederate camp encouraged Brigadier General Charles Pomeroy Stone to order a raid, which clashed with enemy forces.

A prominent U.S. Senator in uniform, Colonel Baker had tried to reinforce the Union troops, but failed to ensure that there were enough boats for the river crossings, which were then delayed.

Baker is killed, and a newly-arrived Confederate unit routs the rest of Stone’s expedition.

The Union losses, although modest by later standards, alarm Congress, which will set up the Joint Committee on the Conduct of the War, a body that will provoke years of bitter political infighting.