Lieber Code
Based on Wikipedia: Lieber Code
The Professor Who Wrote the Rules of War
In April 1863, as the American Civil War raged into its third bloody year, President Abraham Lincoln signed a document that would quietly reshape how the world thinks about warfare. It was called General Orders No. 100, but history remembers it as the Lieber Code. This was the first serious attempt by any modern nation to write down what soldiers could and could not do in war.
The man who wrote it had an extraordinary qualification: he understood war from every possible angle.
Franz Lieber was a German-born lawyer and philosopher who had fought as a young man in the Napoleonic Wars. Later, he joined the Greek War of Independence against the Ottoman Empire. By the time he immigrated to America, he had seen firsthand what armies do when no one tells them what the rules are. He had also seen what happens when they follow rules—and when they don't.
But here's what makes the story remarkable. When Lieber sat down to write the laws that would govern how Union soldiers should treat their enemies, he had three sons fighting in the Civil War. One fought for the Confederacy. Two fought for the Union. The Confederate son was killed at the Battle of Eltham's Landing in 1862.
This wasn't abstract philosophy for Franz Lieber. This was personal.
Why the Union Army Needed New Rules
The existing military regulations dated back to 1806, when warfare looked completely different. Those old Articles of War were written for conventional battles between uniformed armies on open fields. They said nothing about guerrilla fighters. Nothing about civilian collaborators. Nothing about what to do with escaped slaves who had fled to Union lines.
Union commanders faced impossible situations every day. A Confederate guerrilla ambushes your supply wagon and kills three of your men. You capture him. What now? He's not wearing a uniform. He has no chain of command. According to military custom, you could execute him on the spot. But should you? And what about the farmer who hid him in his barn?
Then there was the question of escaped slaves. Congress had passed a law in 1862 forbidding the return of escaped slaves to the Confederacy. So when slaves fled to Union lines, you couldn't send them back. But what were they? Refugees? Contraband? Future soldiers? The old rules offered no guidance.
Major General Henry Halleck, who commanded the Union Army, needed answers. He happened to know a professor in New York who had been giving public lectures on "Laws and Usages of War." That professor was Franz Lieber.
The Code Takes Shape
Halleck first asked Lieber to tackle the guerrilla problem. In 1862, Lieber produced a tract called "Guerrilla Parties Considered with Reference to the Laws and Usages of War." It established three tests that determined whether a captured fighter deserved prisoner-of-war status or could be executed as an unlawful combatant.
First: Does he wear a uniform identifying him as part of a recognized military force? Second: Does he operate within a formal chain of command? Third: Can he take prisoners himself, the way a regular army unit can?
Confederate guerrillas typically failed all three tests. This gave Union commanders clear legal authority to deal with them harshly. But it also established a principle that would echo through history: there are rules, and if you follow them, you get protected. If you don't, you've put yourself outside the law's protection.
Satisfied with this work, Halleck and Secretary of War Edwin Stanton commissioned Lieber to write something far more ambitious. They wanted a complete revision of American military law—one that addressed military necessity, humanitarian obligations, the treatment of prisoners, the rights of civilians under occupation, and every other question the Civil War had raised.
Lieber delivered. A committee of generals reviewed his draft. Halleck edited it to align with the Emancipation Proclamation, which Lincoln had issued on January 1, 1863. On April 24, 1863, Lincoln promulgated General Orders No. 100.
What Military Necessity Actually Means
The heart of the Lieber Code is a concept that still dominates discussions of warfare today: military necessity. It's a phrase that sounds like it could justify anything. The genius of Lieber's formulation is that it does the opposite—it defines exactly what necessity permits and exactly what it forbids.
Military necessity, the Code declared, "consists in the necessity of those measures which are indispensable for securing the ends of the war, and which are lawful according to the modern law and usages of war."
Notice what's happening there. Necessity doesn't override law. Necessity must operate within law. You can't claim military necessity for something that's already illegal.
The Code then lists what necessity does permit. You may kill enemy soldiers. You may capture enemy fighters and anyone "of importance to the hostile government." You may destroy property. You may obstruct roads and communications. You may cut off the enemy's food supply. You may requisition whatever your army needs from enemy territory. You may deceive the enemy through legitimate ruses of war.
But here's the crucial turn. The Code then lists what necessity does not permit—ever, under any circumstances.
Military necessity "does not admit of cruelty—that is, the infliction of suffering for the sake of suffering or for revenge." It does not permit maiming or wounding prisoners. It absolutely forbids torture to extract confessions. It forbids poison in any form. It forbids "wanton devastation"—destruction for its own sake rather than for military advantage.
The Code acknowledges that war involves deception but "disclaims acts of perfidy." The distinction matters. You can wear camouflage. You can set up a fake headquarters to draw enemy fire. You can spread false information. But you cannot abuse a flag of truce. You cannot pretend to surrender and then open fire. You cannot disguise soldiers as medics or chaplains. Those acts would make it impossible to ever trust any enemy signal, which would make peace impossible. And the Code never forgets its ultimate purpose: "military necessity does not include any act of hostility which makes the return to peace unnecessarily difficult."
That last phrase is worth sitting with. Even in the midst of fighting, you must preserve the possibility of reconciliation.
The Problem of Prisoners
Prisoner-of-war status was one of the Civil War's most vexing problems, and the Lieber Code addressed it directly.
The general rule was humane treatment. Prisoners must be fed. They must receive medical care. They must be protected from violence. Killing prisoners was forbidden "except when taking prisoners endangers the capturing unit." That exception acknowledges a hard reality. If you're a small patrol deep in enemy territory and you capture a hundred soldiers, guarding them might get you all killed. The Code doesn't pretend such situations don't exist. But it makes the exception narrow and specific.
The more explosive issue involved race. The Confederate government had proclaimed that captured Black soldiers would not be treated as prisoners of war. They would be treated as escaped slaves—subject to execution or re-enslavement. White officers commanding Black troops would be prosecuted as criminals for helping slaves escape bondage.
The Lieber Code rejected this completely. It prohibited racial discrimination in the treatment of prisoners. A captured soldier was a captured soldier, regardless of skin color. This aligned the military regulations with the Emancipation Proclamation and established that the Union Army would retaliate if the Confederacy mistreated Black prisoners.
This wasn't just a moral statement. It was a practical necessity. By 1863, tens of thousands of Black men were serving in the Union Army. If they faced certain death or enslavement upon capture while white soldiers faced relatively comfortable imprisonment, that disparity would be intolerable. The Lieber Code made clear that the Union would insist on equal treatment—and would take harsh measures if it didn't get it.
Occupation and Resistance
When an army conquers territory, what happens to the people who live there? The Lieber Code proposed what you might call a bargain.
Civilians who cooperated with military authorities would be treated well. Their property would be respected. Their lives would be protected. They would be governed by martial law, but that law would be administered fairly.
Civilians who resisted—especially through guerrilla warfare—would face "imprisonment and death."
This wasn't arbitrary cruelty. It was an attempt to create clear incentives. Cooperate, and military occupation would be tolerable. Resist through irregular warfare, and the consequences would be severe. The Code was trying to channel conflict into predictable, manageable forms. Uniformed armies following rules could eventually make peace. Guerrillas hiding among civilians could keep a war going forever.
The Code permitted retaliation against enemy forces that violated the laws of war. If the Confederates gave no quarter—killing Union soldiers who surrendered—the Union could respond in kind. But retaliation had to be proportional and carefully considered. "Retaliation shall only be resorted to after careful inquiry into the real occurrence," the Code insisted. Hasty retaliation based on rumor or anger was forbidden.
Why? Because "unjust or inconsiderate retaliation removes the belligerents farther and farther from the mitigating rules of regular war, and by rapid steps leads them nearer to the internecine wars of savages." The Code understood how easily tit-for-tat violence can spiral into total brutality. It was trying to keep the Civil War from becoming a war of extermination.
Sherman's March and the Code in Action
Perhaps the most famous application of the Lieber Code came during General William Tecumseh Sherman's March to the Sea in late 1864.
Sherman's Special Field Orders No. 120 explicitly drew on General Orders No. 100. It established a graduated response based on civilian behavior. In areas where the army moved unmolested, soldiers were forbidden from destroying civilian property. But if guerrillas attacked Union forces, if civilians burned bridges or blocked roads, if the population "otherwise manifest local hostility," then commanders should "order and enforce a devastation more or less relentless according to the measure of such hostility."
This has made Sherman controversial ever since. Was his march through Georgia a war crime, or was it a legitimate military operation conducted within the rules?
The Lieber Code suggests the latter—at least in principle. Sherman wasn't engaging in wanton devastation for its own sake. He was applying pressure proportional to resistance. Areas that didn't resist weren't destroyed. Areas that did resist faced consequences calibrated to their level of hostility.
You can argue about whether Sherman's soldiers actually followed these rules or whether they used them as cover for indiscriminate destruction. That's a historical debate that continues today. But the existence of the rules mattered. They established a standard against which conduct could be judged. Before the Lieber Code, there was no such standard.
The Code's Intellectual Ancestry
Franz Lieber didn't create his ideas from nothing. He drew heavily on the Prussian military theorist Carl von Clausewitz, whose book "On War" had become enormously influential in European military circles.
Clausewitz argued that war is not an end in itself. War is politics by other means—a tool for achieving political objectives. This implies that war should be fought in ways that advance those objectives and avoided when it doesn't serve them. It also implies that the ultimate goal of war is a satisfactory peace.
Lieber absorbed this thinking completely. If war's purpose is peace, then you shouldn't conduct war in ways that make peace impossible. If war is instrumental, not existential, then there are limits on what it makes sense to do. You shouldn't poison wells because you want to use those wells after you win. You shouldn't massacre populations because you need to govern those populations later. You shouldn't commit atrocities because atrocities breed hatred that lasts for generations.
This is why Lieber preferred short, decisive wars. A war that drags on for years brutalizes everyone involved. A quick, overwhelming victory preserves both armies and civilians. The Lieber Code legitimized aggressive warfare precisely because aggressive warfare, properly conducted, could end the war faster and with less total suffering than a prolonged stalemate.
From American Law to International Law
The Lieber Code was American law, written for American soldiers fighting an American war. But its influence spread far beyond the United States.
At the Hague Peace Conferences of 1899 and 1907, delegates from countries around the world gathered to codify international laws of war. They used the Lieber Code as a foundation. Its definitions of combatants and non-combatants, its distinction between legitimate military targets and protected persons and property, its rules on occupation and prisoners—all of these flowed into the Hague Conventions that would govern international warfare for decades.
When the world needed to prosecute war criminals after World War II, at Nuremberg and Tokyo, prosecutors argued that by 1939 the laws of war were so well established that no government could claim ignorance. The chain of legal development ran straight back to a German immigrant professor working in New York during the American Civil War.
Franz Lieber's son, Guido Norman Lieber, became the Judge Advocate General of the Army—the military's top legal officer—during the Spanish-American War and the Philippine-American War at the turn of the twentieth century. The Lieber Code governed courts-martial of American soldiers and prosecutions of Filipino resistance fighters. Its reach extended far beyond anything Franz Lieber could have imagined when he wrote it.
The Code's Legacy Today
In 2015, the United States Department of Defense published its Law of War Manual, which was updated in 2023. The manual explicitly references the Lieber Code, and its influence is apparent throughout.
The fundamental framework remains intact. Military necessity permits certain actions but not others. Prisoners must be treated humanely. Civilians must be protected from wanton violence. Torture is forbidden absolutely. These principles, first codified in 1863, still govern how American forces are supposed to conduct themselves in combat.
Whether forces always follow these rules is another question. Every war produces accusations of violations. But the existence of the rules matters. They provide a standard for training soldiers before they deploy. They provide a basis for prosecuting soldiers who commit crimes. They provide a vocabulary for discussing what went wrong when things go wrong.
The Lieber Code was written during a war that killed more Americans than any other conflict in history. It was written by a man whose family was literally torn apart by that war—one son dead fighting for the Confederacy, two others fighting for the Union. It emerged from the recognition that even in the midst of unspeakable violence, some things must remain forbidden. Some limits must be maintained. Some humanity must be preserved.
That recognition—that war can be regulated, that soldiers can be held accountable, that military necessity has boundaries—is Franz Lieber's lasting gift to the world. It's the foundation on which all modern laws of war are built.