There is good reason to think that law and war have nothing to do with one another, and this has certainly been so for most of the lifetime of mankind. Cicero's famous observation--silent enim leges inter arma--from which I take my title, was not a novel insight when uttered in 52 B.C. and in any case was not said in the context of war, but of a prosecution for murder in the aftermath of the Roman riots of that era between the partisans of the populares and optimates. Clausewitz, however, said much the same thing when he decried moderation in warfare, and expressed contempt for legal rules:
War is ... an act of force to compel an enemy to do our will.... [A]ttached to force are certain, self-imposed, imperceptible limitations hardly worth mentioning, known as international law and custom, but they scarcely weaken it. Force--that is, physical force, for moral force has no existence save as expressed in the state and its law--is thus the means of war. (3) This view of law and war as mutually exclusive has prevailed through most of the various periods in the life of the modern state.
The princely states that dominated Europe during the sixteenth century took the view that the practices of war were governed by necessity, and that a war was just if it was fought on just grounds. (4) By contrast, the territorial states that reigned in the eighteenth century recognized that each side usually thought its cause was just. It was an age of cynicism--or realism, if you prefer--and therefore focused instead on evaluating the practices of war themselves. Imperial state-nations, whose hegemony lasted roughly from the American Revolution until the end of the First World War, took yet a different view. Their intense nationalism--let's call it an early form of "exceptionalism"--tended to justify both means and ends, and in so doing, removed from oversight altogether the acts of state once belligerency was begun.
It was from this paradigm that Philip Marshall Brown wrote in 1918 in the pages of the American Journal of International Law:
War is the abandonment of litigation and argument. It is the negation of law.... on the field of battle there is no compelling voice of authority to prevent or to punish violations of the usages and rules of war. If the victor has been guilty of infractions, he suffers no penalty. If the vanquished has been guilty, his offenses are expiated incidentally in the larger penalty of defeat itself. (5) This is the voice of the imperial state-nation, and it holds--as did its champion Clausewitz--that "where the law of peace fails to provide an adequate remedy for international wrongs, it concedes the right of nations to resort to measures of 'self-help'.... The true function of international law is not to regulate war, but to regulate the peaceful relations of states." (6) Once war commences, the law ceases to have effect.
But at the time of this observation, during the First World War (a war that would destroy the constitutional order of imperial state-nations), there was already in being another voice, another constitutional order, that would eventually impose its ideas of law and war on the society of states. In Germany and in the United States, a new constitutional order--the industrial nation-state--was emerging and it had radically different ideas of this relationship.
It might appear paradoxical to state that the first statutory code regulating military behavior was consistent with an increasing ferocity of warfare, but that was, in fact, the contribution of this new form of the state. Its author was an extraordinary man for whose life the birth of this new constitutional world of the nation-state was an animating spirit.
Francis Leiber was born in Berlin and joined the Prussian army at age fifteen. He was wounded shortly thereafter at Waterloo. By the time he sought admission to university he was already well-enough known for his anti-monarchical views to be rejected by the University of Berlin. Hastily moving to the University of Jena he completed his dissertation in mathematics in only four months, just before the authorities caught up with him and he was forced to flee to Dresden. He only briefly resumed his studies because he soon volunteered to serve in the Greek Revolution of 1821. By 1825 he had gone to London, by 1827 he was in Boston, and by the time of the American Civil War he was ensconced as a professor in the Columbia Law School--just in time for the birth of the constitutional order of the industrial nation-state. It was there that, at President Lincoln's request, he produced the Lieber Code--General Orders Number 100--that is the basis for the first codified laws of war.
The Lieber Code is widely admired as the foundation of the law of armed conflict, and it has been copied by many states. It was the basis of the Hague Conventions of 1899 and 1907, of which the Geneva Conventions of our own day are descendants. Yet rarely commented on is the nature of the warfare that the Code permitted--large scale attacks on civilian populations and the leadership of the state. Leiber himself was in charge of the incriminating documents seized after the Civil War that established a Union plot to assassinate the Confederate president and his cabinet; these documents were destroyed when Leiber handed them over to the Republican secretary of war, but fortunately photographic copies had been made.
Twentieth century nation-states fought wars against nations, that is, against national peoples, and so it was not surprising that the national mobilizations of the First World War also brought about a new vulnerability for civilians. The Germans, who seem to have been more aggressive with these tactics than their adversaries, were the innovators, if that is the right word, and it cost them dearly in public opinion, but their model of warfare--not the one that had dominated the Napoleonic Wars--was the model that states copied. Writing in the midst of the First World War, T.J. Lawrence lamented:
[M]odern developments have put in great jeopardy the time-honoured distinction between combatants and non-combatants.... This is due to many causes, most of which are connected with the modern growth in the power and authority of the State.... [War] is not merely a war of governments, but of peoples.... Every ounce of strength each side possesses is thrown into the fight; and the world is beginning to realise for the first time what a war of nations, rather than of armies, means.... Not only can the modern State, with its practical omnipotence, put the bulk of its able-bodied males into the field, but it can organise the rest for supply services.... Its vast armies require artillery and munitions to an extent that was undreamed of even three years ago. To give them what is essential for their operations other armies of engineers, mechanics, and craftsmen of all kinds have to be organised at home.... Is the fighting-man a warrior, and the man or woman who supplies him with the means of fighting a peaceful member of society? ... ... Yet side by side with [this development] other agencies have been at work, with the direct object of making civilians suffer.... Murder, rape, and plunder are no new things in warfare. What is new among civilised Powers is the deliberate planning and encouragement of them as a means of striking terror into the general population, and inducing them to clamour for peace.... ... [I]s it not also possible that [these] atrocities may be copied in other quarters? May they not become precedents for future wars? ... ... [T]he great difficulty would remain untouched. It does not lie in the deliberate resort to brutalities forbidden by the laws and customs of civilised warfare, but in the very nature of war when waged on a large scale under modern conditions. (7) Well, we know what happened next. On average, 16,000 persons died every day of the Second World War, and only a small percentage of these were soldiers. Lawrence had identified precisely the change and its cause: it was a change in the nature of warfare that was changing the nature of the state, and once the state was changed it would wage war in the new way.
How had this happened to law and war in a century--and in an international constitutional order--that was more saturated with legal process than any that had gone before?
The answer is that strategy was driving the development of law, and what strategy demanded was that law become de-coupled from warfare. This was true of constitutional law--where the problems of extended nuclear deterrence defied preexisting legal rules about the commencement of hostilities--and of international law--leading the International Court of Justice to conclude in 1996 that
the threat or use of nuclear weapons would generally be contrary to the rules of international law applicable in armed conflict ... [but that] in view of the current state of international...