The battlefield has always been a place where chaos threatens to overpower humanity. Throughout history, soldiers marched under banners of victory, yet civilians, prisoners, and the wounded often paid the highest price. Before the nineteenth century, the rules of war were largely unwritten, depending more on military custom than on legal obligation. Then, during the American Civil War, one man sought to replace uncertainty with principles that recognized war's harsh realities while insisting that humanity should never disappear.

That man was Francis Lieber, born in Prussia and shaped by the violence of the Napoleonic Wars, Lieber understood that war could never be entirely civilised. Yet he also believed that it should not become an excuse for unlimited cruelty. In 1863, at the request of President Abraham Lincoln's administration, Lieber drafted General Orders No. 100, better known as the Lieber Code. It became the first modern attempt to systematically define the rights and responsibilities of armies during armed conflict.

The Lieber Code acknowledged an uncomfortable truth: war sometimes requires force, destruction, and military necessity. However, it argued that necessity was never unlimited. Violence could only be justified when it served a legitimate military purpose. Torture, unnecessary suffering, cruelty toward civilians, and wanton destruction were incompatible with the principles the Code sought to establish. Prisoners of war deserved humane treatment, civilians were entitled to protection whenever possible, and military commanders were expected to distinguish between lawful military objectives and acts of vengeance.

For Union officers, the Code became a practical manual. It provided guidance on occupations, spies, prisoners, guerrilla warfare, retaliation, and the conduct of military campaigns. Although drafted for one conflict, its influence quickly reached beyond the American Civil War. Military scholars and diplomats recognised that Lieber had transformed centuries of military customs into a coherent legal framework.

As the decades passed, many of Lieber's principles crossed international borders. They influenced the Hague Peace Conferences of 1899 and 1907, which sought to establish internationally accepted rules governing warfare. Later, humanitarian protections developed further through the Geneva Conventions, expanding safeguards for wounded soldiers, prisoners of war, and civilians caught in conflict. The Lieber Code became one of the intellectual foundations upon which modern international humanitarian law was built.

Yet history continued to evolve. The United States Army no longer uses General Orders No. 100 as an active legal manual. After the end of the American Civil War in 1865, the Lieber Code ceased to function as an operational guide for U.S. forces and was gradually replaced by later military regulations and international legal developments. Modern U.S. military operations are governed by contemporary military doctrine, domestic law, customary international law, and treaty obligations, including the Geneva Conventions of 1949 and other applicable agreements. The Lieber Code remains an important historical document rather than a source of binding military authority.

Its greatest achievement, therefore, is not that it continues to govern armies directly, but that it helped reshape how the world thinks about warfare. It introduced the idea that military necessity and humanitarian restraint could exist together, challenging the belief that victory justified unlimited violence. Many of its concepts have been refined, expanded, and incorporated into contemporary international humanitarian law, even though the original document itself has long since been superseded.

Today, the Lieber Code stands as a reminder that law evolves alongside society. What began as instructions for Union soldiers became a milestone in the development of the laws of armed conflict. Although no longer followed as binding law by the United States Army, its influence survives in the legal principles that continue to regulate warfare across the world. Its pages are no longer a military handbook, but they remain a testament to one enduring idea: even in war, humanity must never be forgotten.

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In summary,

The Lieber Code was never a permanent U.S. statute or a treaty obligation.

It was a wartime military instruction issued to Union forces during the American Civil War.

It was issued on 24 April 1863 as General Orders No. 100.

It governed the conduct of Union forces during the Civil War.

After the Civil War ended in 1865, it ceased functioning as an active battlefield manual for U.S. troops.
It was gradually replaced by later U.S. military regulations and international humanitarian law developments.

The U.S. is party to the four Geneva Conventions of 1949 but has not ratified Additional Protocol, or Additional Protocol II (although it recognizes many provisions as customary international law).
The Lieber Code became obsolete as a battlefield manual after 1865, while its principles continued through later law.

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19 pages.