American Civil War: Union forces defeated Confederate troops at the Battle of Shiloh, at the time the bloodiest battle in U.S. history, in Hardin County, Tennessee.

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.

During the American Civil War, the United States Army, the land force that fought to preserve the collective Union of the states, was often referred to as the Union army, the federal army, or the northern army. It proved essential to the restoration and preservation of the United States as a working, viable republic.

The Confederate States Army (CSA), 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 support the rebellion 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 States president, Jefferson Davis (1808–1889). Davis was a graduate of the United States Military Academy, on the Hudson River at West Point, New York, and colonel of a volunteer regiment during the Mexican–American War (1846–1848). He had also been a United States senator from Mississippi and served as U.S. Secretary of War under 14th president Franklin Pierce. On March 1, 1861, on behalf of the new Confederate States government, Davis assumed control of the military situation at Charleston Harbor in Charleston, South Carolina, where South Carolina state militia had besieged the longtime Federal Fort Sumter in Charleston harbor, held by a small U.S. Army garrison under the command of Major Robert Anderson (1805–1871). By March 1861, the Provisional Congress of the Confederate States meeting in the temporary capital of Montgomery, Alabama, expanded the provisional military forces and established a more permanent regular Confederate States Army.