The Lieber Code (General Orders No. 100, April 24, 1863) was the military law that governed the wartime conduct of the Union army by defining and describing command responsibility for war crimes and crimes against humanity; and the military responsibilities of the Union soldier fighting in the American Civil War (April 12, 1861 – May 26, 1865) against the Confederate States of America (February 8, 1861 – May 9, 1865). The General Orders No. 100: Instructions for the Government of the Armies of the United States in the Field (Lieber Code) were written by Franz Lieber, a German lawyer, political philosopher, and combat veteran of the Napoleonic Wars.

At military age, the jurist Francis Lieber soldiered and fought in two wars, first for Prussia in the Napoleonic Wars (18 May 1803 – 20 November 1815) and then in the Greek War of Independence (21 February 1821 – 12 September 1829) from the Ottoman Empire (1299–1922). In his later career, Lieber was an academic at the College of South Carolina, in the southern region of United States of America. Although not personally an abolitionist, Lieber opposed slavery in principle and in practice because he had witnessed the brutalities of black chattel slavery in the South, from which he departed for New York City in 1857. In 1860, Professor Lieber taught history and political science at the Columbia Law School, and publicly lectured about the "Laws and Usages of War" proposing that the laws of war correspond to a legitimate purpose for the war.

During that time, Lieber had three sons who fought in the American Civil War (April 12, 1861 – May 26, 1865): one in the Confederate Army, who was killed at the Battle of Eltham's Landing (May 7, 1862), and two in the Union army. Later in 1862, in St. Louis, Missouri, while searching for the Union-soldier son wounded at the Battle of Fort Donelson (February 11–16, 1862), Lieber asked the help of his professional acquaintance Major General Henry W. Halleck, who had been a lawyer before the Civil War and was the author of International Law, or, Rules Regulating the Intercourse of States in Peace and War (1861), a book of political philosophy that emphasized legal correspondence between the casus belli and the purpose of the war.

In fighting the Confederate Army, guerrillas, and civilian collaborators of the Confederacy, Union army soldiers and officers faced ethical dilemmas of command responsibility concerning their summary execution in situ, per military custom, because the 1806 Articles of War did not address the management and disposition of prisoners of war and irregular fighters; nor the management and safe disposition of escaped black slaves – who were not to be repatriated to the Confederacy, per the Act Prohibiting the Return of Slaves (1862).

To resolve the lack of military authority in the 1806 Articles of War, Halleck, the Commanding General of the Union army, commissioned Lieber to write military laws specific to the modern warfare of the American Civil War. For the Union army's management and disposal of irregular fighters (guerrillas, spies, saboteurs, et al.), Lieber wrote the tract of military law Guerilla Parties Considered with Reference to the Laws and Usages of War (1862), which disallowed a soldier's POW-status to Confederate guerrillas and irregular fighters with three functional disqualifications: (i) guerrillas do not wear the army uniform of a belligerent party to the war; (ii) guerrillas have no formal chain of command, like a regular army unit; and (iii) guerrillas cannot take prisoners, as could an army unit.

At the end of 1862, General Halleck and War Secretary Stanton commissioned Lieber to revise the military law of the 1806 Articles of War to include the practical considerations of military necessity and the humanitarian needs of civilian populations under military occupation. The editorial-revision committee, Major General Ethan A. Hitchcock and Major General George Cadwalader, Major General George L. Hartsuff and Brigadier General John Henry Martindale, requested from Lieber comprehensive military laws to govern the Union army's prosecution of the Civil War. Gen. Halleck edited Lieber's military law to concur with the Emancipation Proclamation (1 January 1863), and, on April 24, 1863, President Lincoln promulgated General Orders No. 100, Instructions for the Government of the Armies of the United States in the Field, the Lieber Code.

For the Union army's prosecution of the American Civil War, General Order No. 100 (April 24, 1863) concerns the practical particulars of martial law, military jurisdiction, and the treatment of Confederate irregular fighters, such as spies, deserters, and prisoners of war. In the field practice of military justice, the unit commander held authority for any prosecution under the Lieber Code, which command authority included the summary execution of Confederate prisoners of war and war-criminal soldiers of the Union army. In the context of the American Civil War, the Lieber Code explains the concepts of military necessity and humanitarian needs in articles 14, 15, and 16 of Section I:

In the late 19th century, the Lieber Code was the first modern codification of both customary international law and the law of war of Europe, and later was a basis for the Hague Convention of 1907, which restated and codified the practical particulars of that U.S. military law for application to international war among the signatory countries.