He became commandant of cadets at West Point and taught infantry, artillery, and cavalry tactics. While teaching here, he wrote a book,"Rifle and Ligbt Infantry Tactics," or more familiarly "Hardee's Tactics," became the standard textbook and was widely used by both sides during the Civil War.
While assigned as a Lieutenant Colonel in the 1st Cavalry, he resigned his U.S. Army commission on January 31, 1861, 12 days after his home state of Georgia seceded.

When he joined the Confederacy, he was commissioned as a Colonel. On June 17, he was promoted to Brigadier General. He organized and commanded the Arkansas Brigade and then transferred to Kentucky in the fall of 1861 to command his own brigade. Once in Kentucky, he was promoted to major general on October 7 and given command of a corps.

He led his corps in the battles at Shiloh (being wounded), Perryville, Murfreesboro, Chickamauga, Lookout Mountain, Missionary Ridge, and the Atlanta Campaign after being promoted to lieutenant general on October 10, 1862. As one of the original lieutenant generals allowed under Confederate law, he led an official corps at Murfreesboro and during the Tullahoma Campaign.

In order to get away from the despised army commander, Gen. Braxton Bragg, he took an assignment in Mississippi under Gen. Joseph E. Johnston. He was transfered to command of the Atlantic coast and served there for the balance of the war. He was unable to stop Sherman's "March to the Sea", but on December 18, 1864, he successfully evacuated Savannah at the last minute. His last fight was at Bentonville, where he saw his only son killed in battle.

He surrendered his force, along with Gen. Joseph E. Johnston's command, on April 26, 1865.
Hardee would earn the nickname of "Old Reliable" by being an outstanding corps commander.
After the war, he settled on an Alabama plantation and became a planter.

Promotions:

• Lieutenant Colonel - February ??, 1861
• Brigadier General - June 17, 1861
• Major General - October 7, 1861
• Lieutenant General - October 10, 1862

Major Commands:

• Upper District of Arkansas, Department #2 (July 22 - October 1861)
• lst Division, Central Army of Kentucky, Department #2 ( October 28 - December 18, 1861, and February 23 - March 29, 1862)
• III Corps, Army of the Mississippi (March 29 - July 5, 1862)
• II Corps, Army of Tennessee (November 20, 1862 - July 14, 1863)
• Army of the Department of Mississippi and East Louisiana (July 14-November 1863)
• Department of South Carolina, Georgia and Florida (October 5, 1864 - February 16, 1865)