Theorists often distinguish between two ways to conceptualize the law of war – jus ad bellum, the law of appropriate justifications for going to war, and jus in bello, the law of acceptable behavior in war. Although not mutually exclusive, often in history one or the other framework has been predominant. For example, in medieval Europe jus ad bellum reigned, as Catholic theologians such as Augustine of Hippo and Thomas Aquinas developed and disseminated just war theory. The Middle Ages were indeed a time of rape and pillage, but what mattered more than actual conduct in battle was having the just or Godly cause. In the late nineteenth century, for a number of philosophical, technological, and social reasons, jus in bello was prioritized and the modern foundations of the law of war were set – although some of these achievements proved illusory.

Historians have long described positivism as being the dominant legal paradigm of the nineteenth century, in part as a response to widespread attack on traditional social institutions during the French Revolution based on natural legal principles, which utilitarian philosopher Jeremy Bentham called “nonsense upon stilts." Bentham, who also coined the term “international law” to replace the antiquated “law of nations,” in turn greatly influenced a later jurist who renounced the idea of international law almost entirely, English jurist John Austin. Austin believed that “[l]aws properly so called are a species of commands. But, being a command, every law properly so called flows from a determinate source,” which was conspicuously lacking in the international realm. However, another, more subtle byproduct of the French Revolution and the Napoleonic Wars, had been to inculcate to a new generation of statesmen and military leaders the primacy of the power of the state. The power of a state – unfettered from religious, ideological, or moral constraints – in international relations therefore led it to be the sole and thus “determinate source” of international law, that law being only those obligations states voluntarily agreed to. Bolstering this philosophy of legal positivism were also the economic – laissez-faire – and scientific ethos of the age: lawyer members of the Association internationale pour le progrès des sciences sociales founded a scientific legal journal, the Reuuede droit international et de législation comparée in 1868 and the first on the subject of international law, under the assumption that the full extent of the law could be expressly enumerated like the laws of chemistry.

Nowhere were these trends more clear than in the law of war. The great student of Napoleon, Carl von Clausewitz, called war “a political instrument, a continuation of political relations, a carrying out of the same by other means." War was therefore seen in the nineteenth century as both a political tool and a legal institution, devoid of a morality of its own and only subject to the constraints to which states consented.

Meanwhile, technological advances and social change had a profound effect on war during the nineteenth century. Industrial revolution innovations like the railway and telegraph were first used to deadly effect in the Crimean War, and the latter allowed for gory details of the suffering of soldiers to be reported to an increasingly literate and opinionated public, almost “live,” in mass-circulation newspapers like The Times by a first generation of “war correspondents.” Florence Nightingale tried to mitigate some of this suffering in her pioneering nursing efforts, but it was not until a lesser-known but more destructive war that people were motivated to create a legal framework to alleviate battlefield suffering.  Soon the changes rendering war more heinous, that knowledge being made widespread, and the ability of states as the only actors in international law to create rules of war, came together.

During the Wars of Italian Unification, pitting Piedmont-Sardinia and France against Austria, the Battle of Solferino was fought on 24 June 1859. Not since the Napoleonic Battle of Leipzig (also known as the Battle of Nations for its many combatants) had so many men taken part; not since the Battle of Waterloo had there been so many casualties. And yet, medical services were practically nonexistent: the French cavalry had four veterinarians for every 1000 horses but only one doctor for every 1000 soldiers. A 31-year old Swiss civilian, Henry Dunant, was on business in Italy at the time and witnessed firsthand the horror of the battle and its aftermath for wounded soldiers. Profoundly affected, he published A Memory of Solferino at his own expense and he personally sent the work to heads of state, doctors, and other people of influence. Together with another prosperous Swiss businessman, Gustave Moynier, Dunant went on to found the International Committee for Relief to the Wounded, which had it first meeting, attended by representatives from sixteen countries, in October 1863. A year later, twelve states signed the first Geneva Convention "for the Amelioration of the Condition of the Wounded in Armies in the Field." The International Committee, which adopted the Red Cross as its symbol, became a neutral, intermediary body to watch over these new and specific rules of war to which states had voluntarily bound themselves. In 1901 Dunant was a joint winner of the first Nobel Peace Prize. Today, the Geneva Conventions consist of four multilateral treaties negotiated in 1906, 1929, and 1949 and still constitute the base of international humanitarian law.

The First World War put into question the value of much of this law. Total war casualties reached a harrowing 38 million people, or almost 52 percent of all combatants. On first day of Battle of the Somme, Britain suffered 60,000 casualties alone. In this context, jus in bello came to seem woefully insufficient. The International Committee of the Red Cross was active throughout the war and its aftermath, organizing the exchanges and return of POWs, for example, or protesting the use of chemical weapons – and was awarded the 1917 Nobel Peace Prize, the only such award during the war, for its extraordinary if largely ineffectual efforts. However, the colossal destruction of the war, exacerbated by the postwar perception that the slaughter had largely been futile, inclined lawyers and statesmen to turn their attention if not away from jus in bello entirely, then to it in conjunction with jus ad bellum – and from there the modern attempt to make the very waging of war illegal was born.