War’s exponential growth in size and destructiveness triggered the first efforts at regulation. During the Civil War, President Lincoln issued General Orders No. 100, the Instructions for the Government of Armies of the United States in the Field—the first official public code of the laws of war. Article 15 of the Code set out the wide means available to nations at war:
Military necessity admits of all direct destruction of life or limb of armed enemies, and of other persons whose destruction is incidentally unavoidable in the armed contests of the war; it allows of the capturing of every armed enemy, and every enemy of importance to the hostile government, or of peculiar danger to the captor; it allows of all destruction of property, and obstruction of the ways and channels of traffic, travel, or communication, and of all with-holding of sustenance or means of life from the enemy; of the appropriation of whatever an enemy’s country affords necessary for the subsistence and safety of the army, and of such deception as does not involve the breaking of good faith either positively pledged, regarding agreements entered into during the war, or supposed by the modern law of war to exist. Men who take up arms against one another in public war do not cease on this account to be moral beings, responsible to one another and to God.42
Following this logic, an army at war could impose blockades and sieges and even bombard a city, so long as its reduction had military value. Still, Lincoln’s General Orders No. 100 also put into written terms the need to shield civilians, where possible, from the harshness of war. Article 22 declared that the Union armies would respect a distinction between “the private individual belonging to a hostile country and the hostile country itself, with its men in arms.” But civilian immunity from hostilities would run only as far as military necessity allowed. “The unarmed citizen is to be spared in person, property, and honor as much as the exigencies of war will admit.”
Francis Lieber, a Prussian immigrant and advisor to the Lincoln administration, drafted the rules. He believed that the modern age had made war into a contest between mass armies rather than individuals.43 Lieber did not think that the customs of international law should ban most methods of war, so long as the destruction was no “greater than necessary.” Accepting conflict as a permanent feature of international affairs, Lieber believed that fiercer wars were more humane because they were shorter, an idea shared by Clausewitz and Machiavelli. Therefore, the laws of war allowed almost any destruction that advanced the goals of the war, and the use of “those arms that do the quickest mischief in the widest range and in the surest manner.”44
The Industrial Revolution and the rise of mass production equipped armies of draftees with highly lethal, yet relatively cheap, standardized weapons. In World War I, rifles accurate over long distances became commonplace. The British Lee-Enfield rifle could fire twelve rounds per minute with accuracy at 600 meters from a ten-round magazine; the British Vickers machine gun could fire 450-600 rounds per minute at a range of 4,000 meters. Artillery became much more significant due to the larger number of pieces, their range and accuracy, and the use of high-explosive shells. Airplanes and tanks made their first appearance in World War I. Dreadnoughts used oil engines, displaced 16,000 tons, and mounted fifteen-inch guns that could hit targets twenty miles away. Submarines entered into widespread use for the first time. Western armies unleashed the first weapons of mass destruction, chemical and biological agents that killed or incapacitated on the battlefield.
Modern industrial production, transportation, communication, and logistics produced even larger armies. By the end of World War I, Russia had mobilized about 12 million soldiers, Germany 11 million, Great Britain 8.9 million, France 8.4 million, Austria-Hungary 7.8 million, Italy 5.6 million, and the U.S. 4.35 million. Modern weaponry’s longer range and destructiveness gave the advantage to defensive warfare in trenches, which inflicted staggering casualties on these new large armies. In the Battle of the Somme, from July 1 to November 18, 1916, both sides suffered more than 1 million killed or wounded—the British Army lost 57,470 on the first day alone, the worst day in its history. Overall casualties dwarfed any previous war in human history: the Allied Powers lost 5 million killed, 12.8 million wounded; the Central Powers lost 8.5 million killed, 21 million wounded. By comparison, in the Napoleonic Wars, France lost 371,000 killed, 800,000 wounded. Its allies lost similar numbers: the British, 312,000; Austria 376,000; Russia 289,000, and Prussia 134,000. Efficiency did not stop with the production of consumer goods; it extended even to the business of killing.
World War II exploited transportation advances to expand the use of air power and armored vehicles, which returned the combat advantage to the offense. Casualty levels climbed again, but within World War I ranges: the Soviet Union lost about 9 million in combat deaths, Germany 5 million, Japan about 2.5 million, the U.S. 407,000, the United Kingdom 384,000, with total worldwide combat deaths ranging from 21-25 million. But World War II witnessed a phenomenon that perhaps had not appeared since the Thirty Years War: massive civilian deaths (about 30 million) in numbers that exceeded military ones. The advent of the atomic bomb at the war’s end raised the specter of even greater civilian casualties in the future.
International law could not stop the spread of technological progress to the machines of war. This has been the lesson of history. Lieber’s Code did not prohibit the Union blockade of the South, the burning of Atlanta, or Sherman’s march to the sea, nor did it prevent the introduction of new weapons such as modern rifles, trenches, and artillery. In World War I, the Allies demanded that German submarines allow ships the opportunity to off-load civilians, which also gave up the element of surprise. Germany ultimately refused, which handed Woodrow Wilson the official rationale to bring the United States into World War I on the side of Great Britain and France. The Washington Naval Conference of 1922 sought to limit large battleships and maintain a rough balance of maritime power between the great powers, but Japan evaded the rules while the locus of sea power shifted instead to aircraft carriers. Nations used chemical weapons in World War I, signed a treaty to ban them in 1925, but have used them in conflicts since.
We are not arguing against all forms of cooperation during armed conflict. Nations have applied general custom to limit the use of weapons that cause unnecessary suffering or superfluous damage and destruction, depending on the factual context. The general principles of Lieber’s Code still guide the conduct of war, even as new technologies transform weapons, tactics, and strategy. Nations have the freedom to use military force in ways that advance the objectives of the war, so long as they minimize harm to civilians as best they can.
Instead, we question the idea that nations should look to formal treaties and rules to produce lasting limits on war. Despite the recent deterioration in the Syrian civil war, nation-states have generally refrained from the use of chemical weapons against each other since the end of World War I. They have followed the Geneva Conventions on prisoners of war, though not consistently. Nations have observed others norms in the breach, chief among them the immunity of the civilian population and resources from attack. World War II not only saw the aerial bombing of cities and the nuclear attacks on Japan, but the years since have seen precision targeting of terrorists off the battlefield, attacks on urban infrastructure, and the acceptance of high levels of collateral damage among civilians. International lawyers and diplomats may proclaim that nations follow universal rules, either because of morality or a sense of legal obligation, but the record of practice tells a far different story. Efforts to impose more specific and demanding rules, such as limiting targeted drone attacks, banning cyber attacks, or requiring human control of robotic weapons, will similarly fail because they cannot take into account unforeseen circumstances, new weapons and military situations, and the immediate exigencies of war. Just as new technology led to increases in economic productivity, so too has it allowed nations to make war more effectively.
Nations