For most of history, the limits on war lived in custom, religion and the honor codes of warriors. In the nineteenth century they began to be written down as law.

The Lieber Code

During the American Civil War, the Union army asked Francis Lieber, a German-born professor at Columbia College in New York, to draft rules for its soldiers. His family embodied the war's tragedy: one son was killed fighting for the Confederacy, and another lost an arm fighting for the Union. President Lincoln approved the result as General Orders No. 100 in April 1863. The Lieber Code was the first modern codification of the laws of land war.

Its most quoted line limits the idea of necessity: "Military necessity does not admit of cruelty, that is, the infliction of suffering for the sake of suffering or for revenge, nor of maiming or wounding except in fight, nor of torture to extort confessions." Yet the Code was a document of hard war, not soft war. The Lieber Code also permitted starving the enemy, armed or unarmed, to hasten defeat.

Solferino and the Red Cross

On 24 June 1859, French and Sardinian armies fought the Austrians at Solferino in northern Italy. A Swiss businessman, Henry Dunant, arrived as the fighting ended and found thousands of wounded men lying untended in the summer heat. He organized local villagers to bring water and bandages, and they helped soldiers of both sides. In 1862 he published A Memory of Solferino. Dunant's book proposed national relief societies and an international agreement to protect the wounded.

A committee formed in Geneva in 1863 to pursue his ideas; it became the International Committee of the Red Cross. The next year, twelve states signed the first Geneva Convention. The 1864 Geneva Convention protected wounded soldiers and the medical staff caring for them, who were to be marked by a red cross on a white ground. In 1901 Dunant shared the first Nobel Peace Prize.

Weapons and the public conscience

In 1868, at Russia's invitation, states meeting in St Petersburg agreed to renounce explosive bullets weighing under 400 grams. Their declaration stated the purpose of war in a single sentence: "the only legitimate object which States should endeavour to accomplish during war is to weaken the military forces of the enemy." The St Petersburg Declaration called weakening the enemy's military forces the only legitimate object of war. From that, it reasoned that weapons that uselessly aggravate the suffering of disabled men are contrary to humanity.

Conferences at The Hague in 1899 and 1907 produced fuller rules on fighting, occupation and prisoners. They forbade, among other things, declaring that no quarter will be given. The Russian delegate Fyodor Martens proposed a sentence for the 1899 convention that has since become famous as the Martens Clause. It says that in cases not covered by the rules, "populations and belligerents remain under the protection and empire of the principles of international law, as they result from the usages established between civilized nations, from the laws of humanity, and the requirements of the public conscience." The Martens Clause holds that where treaties are silent, the laws of humanity and the public conscience still bind. Its point is that a gap in the written law is not a license.

After 1945

The four Geneva Conventions of 1949 rewrote the rules for the wounded, for those at sea, for prisoners and for civilians. They are now among the most widely accepted treaties in existence: every state in the world is party to the 1949 Geneva Conventions. Two additional protocols in 1977 strengthened protection for civilians and extended rules to civil wars, though some major powers, including the United States, have not ratified the first of them.

Does any of this work? Skeptics point to the steady stream of violations in every war. Defenders reply that compliance is invisible: prisoners who are fed, hospitals not bombed and surrenders accepted do not make headlines. The laws of war therefore depend so heavily on armies training their own soldiers and on citizens caring whether their side obeys.

Worked example: banning landmines

Antipersonnel landmines are cheap, and they wait. Long after wars in Cambodia, Angola and Afghanistan ended, mines kept killing and maiming farmers and children. In 1992 a group of aid and human rights organizations founded the International Campaign to Ban Landmines. In October 1996 Canada's foreign minister, Lloyd Axworthy, challenged governments to return within a year and sign a ban. They did: the treaty was signed in Ottawa in December 1997, and the campaign and its coordinator, Jody Williams, received that year's Nobel Peace Prize.

The campaign drew on the ideas of this level. A buried mine cannot tell a soldier from a child, so it fails the test of distinction, and its wounds cause suffering long after any military purpose is served. Campaigners appealed directly to public conscience, as the Martens Clause invites. Yet the limits of law showed too. The United States, Russia and China stayed outside the treaty, and in 2025 several states bordering Russia, including Finland, Poland and the three Baltic states, announced that they would withdraw, arguing that their security now required mines. Whether restraint can survive renewed fear is the test such treaties now face.