By early April 1861, Fort Sumter, an isolated Union outpost in Charleston Harbor, South Carolina, was surrounded by Confederate forces and facing starvation. President Lincoln attempted to avoid war by sending only life-sustaining aid to the fort, but the Confederates, eager for conflict, fired upon the fort on April 12. The Union garrison, led by Major Robert Anderson, surrendered after 33 hours of bombardment.

In the North, the attack on Fort Sumter provoked a swift reaction. Lincoln called for 75,000 volunteers to serve for 90 days, aiming to quickly subjugate the South. Virginia and several other states seceded, bringing the Confederacy to 11 states with 5.5 million free citizens and 3.5 million enslaved people.

Meagher, initially sympathetic to the South, quickly sided with the Union following the attack. He joined the 69th New York State Militia, led by Michael Corcoran, and began rallying support and recruiting volunteers, emphasizing the duty of Irish immigrants to defend their adopted country. Meagher formed the Irish Zouaves, named after elite European soldiers, and paraded them through New York to rouse support.

The 69th Militia trained in Washington, DC. Facing prejudice and being considered inferior soldiers, the Irish volunteers were determined to prove their worth. Union troops, under General Irvin McDowell, planned to march on Richmond, with the first objective being the strategic rail junction at Manassas.