During the American Civil War, Ulysses S. Grant led the Union Army. Grant had already established himself as a battlefield leader by the commencement of the war effort. During the Mexican-American War, from 1846 to 1848, he received commendations for heroism and distinguished performance as a captain. When the Civil War broke out in 1861, Grant stood up once more to volunteer to fight for the Union against the Southern Confederate soldiers. Grant began the war as colonel of the 21st Illinois Infantry, but within months of the war's commencement, he was promoted to brigadier general, and then, in July 1861, to commander of the District of Southeast Missouri.

Following Union wins in Tennessee at Fort Henry and Fort Donelson, Grant rose to prominence as a military leader. Grant was given the press moniker "Unconditional Surrender Grant," but his fame was fleeting. The deadly Battle of Shiloh in Tennessee was the first time Grant's military leadership abilities were called into question. By the end of the engagement, 13,000 Union fighters and 10,000 Confederate soldiers had been killed. Military officers demanded that Grant be removed from leadership in a letter to President Lincoln.

President Lincoln refused to dismiss Grant, and further victories at Vicksburg, Mississippi, allowed Union forces to restrict Confederate supply routes vital to the Confederacy's war operations. By March 1864, President Abraham Lincoln had promoted Grant to lieutenant general and appointed him commander-in-chief of the United States Armies. On April 9, 1865, Robert E. Lee, Confederate commander of the Army of Northern Virginia, surrendered to General Grant at Appomattox Court House in Virginia. Following Lincoln's death, President Andrew Johnson appointed General Grant to the position of Secretary of War. Grant went on to defeat President Johnson in the 1868 election and become the 18th President of the United States.Grant presided over the closing years of the Reconstruction era in American culture during his presidency.

During the Civil War, another notable Northern general was William Tecumseh Sherman. Sherman was not born in the South, but his experiences stationed in Florida, Georgia, and South Carolina between 1840 and 1846 gave him a unique understanding of the landscape and people of the region. Due to his gallantry and leadership on the battlefield while serving under Ulysses S. Grant at the Battle of Shiloh, Sherman's rank was raised to major-general. During the Civil War, Sherman and Grant had an extremely tight working relationship. Sherman had been assigned with the hard war military strategy to end the war effort by the end of the war effort. Sherman's army of 60,000 troops marched and conquered territories in Georgia before moving on to seize key locations in South Carolina and North Carolina, destroying infrastructure and personal property in the process. Ultimately, General Sherman's destruction of the South's infrastructure would be pivotal in bringing the Civil War to a close.

President Lincoln first chose George B. McClellan as the Union Army's General-in-Chief. General McClellan launched a major invasion of Virginia known as the Peninsula Campaign in early 1862. The Peninsula Campaign was a significant setback for General McClellan in the early days of the war effort. McClellan underestimated the number of Confederate forces he would confront during the early days of the Peninsula Campaign, which, combined with his troops' delayed march through the region, resulted in strategic flaws that led to the failure of his plans. General McClellan believed that various members of the Washington, DC political class were undermining his success. Due to McClellan's failures during the Peninsula Campaign, Confederate officers such as Stonewall Jackson and Robert E. Lee rose to prominence as crucial commanders of the Confederate war effort. General McClellan eventually lost favor with President Lincoln. General McClellan opposed President Lincoln's goal of abolishing slavery, preferring to preserve the system of slavery in the South. During the 1864 presidential election, McClellan voted against President Lincoln's reelection. President Lincoln deposed McClellan as General-in-Chief of the Army and subsequently Commander of the Army of the Potomac, and after Lincoln's reelection, McClellan never returned to military service.