- December 1861
- October 24, 1862
- During the Civil War, there were two armies designated as the Army of the Ohio.
- The first Army of the Ohio fought in the Western Theater during 1861 and 1862.
- On May 3, 1861, Washington officials issued General Orders Number 14, which created a new military unit known as the Department of the Ohio, consolidating regiments from the states of Ohio, Indiana, and Illinois.
- On May 3, 1861, Washington officials named Major-General George B. McClellan to command the newly created Department of the Ohio.
- Brigadier-General Ormsby M. Mitchel commanded the Department of the Ohio from September 19 to November 15, 1861.
- By May 28, 1861, enough pro-Union regiments had been raised in the State of Kentucky to prompt the U.S. war department to organize the Department of Kentucky, which included as much of the state of Kentucky that lay within 100 miles of the Ohio River.
- Brigadier-General Robert Anderson was the first commander of the Department of Kentucky.
- On August 15, 1861, the war department effectively replaced the Department of Kentucky with the Department of the Cumberland.
- On October 7, 1861, the war department placed Major-General William T. Sherman in command of the Department of the Cumberland.
- On November 9, 1861, the war department issued General Orders Number 37, which dissolved the Department of the Cumberland and expanded the Department of Ohio to include the states of Ohio, Indiana, Michigan, Tennessee, and Kentucky east of the Cumberland River.
- On November 9, 1861, Brigadier-General Don Carlos Buell was selected to command the Department of Ohio.
- In December 1861, Brigadier-General Don Carlos Buell had organized the forces under his command into five divisions that would henceforth be known as the Army of the Ohio.
- On January 19, 1862, the Army of the Ohio defeated Confederate troops commanded by Major- General George B. Crittenden at the Battle of Mill Springs.
- On February 25, 1862, the Army of the Ohio marched into Nashville unopposed, making the Tennessee capital the first Confederate state capital to fall into Union hands during the Civil War.
- On March 11, 1862, President Lincoln issued War Order Number 3, which consolidated three western departments, including the Department of the Ohio into the Department of the Mississippi, commanded by Major-General Henry Halleck.
- On April 6, 1862, the Army of the Ohio reinforced Major-General Ulysses S. Grant’s Army of the Tennessee on the first day of the Battle of Shiloh.
- The Army of the Ohio comprised the center of Major-General Henry Halleck’s forces during the Siege of Corinth (April 29 to May 30, 1862).
- The Army of the Ohio opposed the Army of Mississippi and the Army of East Tennessee during Confederate General Braxton Bragg’s Confederate Heartland Offensive (August–October, 1862).
- The Army of the Ohio fought Confederate forces to a draw at the Battle of Perryville (October 8, 1862).
- On October 24, 1862, the war department issued General Orders Number 168, which created the Department of the Cumberland, encompassing all of Tennessee east of the Tennessee River, plus any parts of northern Alabama and Georgia that might be captured United States troops.
- On October 24, 1862, the war department issued General Orders Number 168, which placed Major-General William Rosecrans in charge of all U.S forces within the newly created Department of the Cumberland and relieved Major-General Don Carlos Buell of his command.
- On October 24, 1862, the war department issued General Orders Number 168, which designated all forces within the newly created Department of the Cumberland as the 14th Corps; the 14th Corps soon became known as the Army of the Cumberland, thus ending the existence of the Army of the Ohio.