Nor have these new weapons technologies yet sparked a useless arms race. Nuclear weapons eventually became opportune for arms control because larger stockpiles provided marginal, if any, benefits due to the destructive potential of each weapon and the deterrence provided by even a modest arsenal. Mutual reductions could leave both sides in the same position as they were before the agreement. Today, the marginal cost of nuclear weapons for the U.S. and Russia so outweighs their marginal benefit that it is not even clear that a binding international agreement is needed to reduce their arsenals. Russia, for example, reduced its arsenal below New START’s ceilings of 1,550 nuclear warheads and 700 strategic launchers even before the U.S. approved the deal.45 The United States likely would have reduced its forces to those levels even if the Senate had refused to consent to the treaty, a position the executive branch also took in 2002 with the Treaty of Moscow’s deep reduction in nuclear weapons. Today’s new weapons do not yet bear these characteristics. The marginal gains in deploying these weapons will likely be asymmetric across nations insofar as some nations will experience much greater gains in military capability by developing cyber and drone technology. Put differently, prohibition or regulation of these new weapons will not have equal impacts on rival nations. Indeed, we do not even now have enough information to understand which nations will benefit and which will not, which makes any form of international ban even less likely.
Nuclear weapons are the exception that proves the rule. Their unique characteristics and deterrent value make them suitable for international cooperation to limit their use. But the twentieth century has otherwise shown that technological advances, and the increases in military effectiveness that have followed, have outpaced law. Efforts to prevent the introduction of new weapons have failed because the weapons themselves initially advantage early adopters. Legal regulation will not emerge until nations have gained significant information about how the technology and its constraints on its use may affect them. In the absence of specific agreements, nations will still follow the customary rules of war, which provide general principles of reasonableness to apply to new circumstances, such as traditional prohibitions against wanton destruction or unnecessary suffering. It is to the new world of war that we now turn.
Static Law for a Changing World?
The laws of war have not kept pace with the rapid change in weapons technology. Efforts to freeze war in place by adopting an inflexible legal approach may lead to a failure of the current framework of war or its rewriting by nations less friendly to the Western international order. Another set of changes in war, also spurred by changes in politics, economics, and technology, is placing further pressure on the idealized AP I and U.N. Charter vision of the international order. In this new century, the classic paradigm of war between nation-states with disciplined militaries has slowly given way to a more chaotic world in which terrorist organizations, regional guerrillas, and ethnic or religious groups conduct equally violent hostilities. The great majority of casualties now come from civil wars and disputes within states, rather than wars between states. Today, the great powers use their militaries to threaten or intervene against smaller states, rather than in direct battles against each other. To grapple with these problems, the laws of war should allow both the use of new weapons more widely and the use of force more often.
The geopolitical order of the nineteenth century was determined largely by the military and economic strength of nations, not international agreements. Though legal treatises at that time still embraced the traditional view that a just war requires a legitimate causus belli, many causes were regarded as “good” and there was little enforcement of the just war requirement. Nations often went to war to enlarge their territory, as the United States did in the Mexican War of 1846-48, or to prevent others from expanding their influence, as when Great Britain and France fought Russia in the Crimean War of 1853-56. European nations constructed a Concert of Europe to strike a balance between the great powers, with war as the final mechanism to ensure that no state grew too powerful. Even when a state invoked transparently contrived claims to justify war for other reasons, as Bismarck’s Prussia did to unify Germany, no outside power helped the victims. Wars were brief in time and limited in scope.
The breadth of destruction wrought by World War I, however, prompted nations to attempt a rewrite of the international order. Rather than a balance of power and contending alliances, nations would guarantee their security in a peace treaty that established a scheme for “collective security” to guarantee every state its “political independence and territorial integrity” against aggression.46 They established an international forum to help resolve disputes, the League of Nations, then tried to outlaw war in the Kellogg-Briand Pact, allowing (supposedly) a resort to force only in self-defense. But the League failed to take effective action in response to either Japanese aggression against China or Italian aggression against Ethiopia. It could not draw in the two most powerful nations in the world, the United States and the Soviet Union, to cooperate to maintain international order. When Britain and France declared war in response to Germany’s aggression against Poland, no one even bothered to consult the League.
After the failure of the League to keep the peace, the victors renewed and extended their commitment to collective security. Maintaining the League’s guarantee of political independence and self-determination for all member states, the United Nations Charter banned war except in cases of self-defense. Force would remain the province of the Security Council, along with coercive measures short of war, such as economic sanctions. Article 42 declares that the Security Council “may take such action by air, sea, or land forces as may be necessary to maintain or restore international peace and security. Such action may include demonstrations, blockade, and other operations by air, sea, or land forces of Members of the United Nations.”47 Article 51, however, contained the great exception for self-defense: “Nothing in the present Charter shall impair the inherent right of individual or collective self-defence if an armed attack occurs against a Member of the United Nations, until the Security Council has taken measures necessary to maintain international peace and security.”48
Article 51 itself does not define self-defense. Nonetheless, well-regarded legal commentators insist that the U.N. Charter allows force only when the Security Council cannot intervene successfully to counter a cross-border invasion.49 This logic mimics domestic criminal law, which allows victims to physically resist a threat of deadly harm only if the police cannot prevent the violence. This doctrine does not, however, reflect the practice of most U.N. states. States have used armed force against each other hundreds of times since 1945. The Security Council has authorized only a handful of them due to the veto power of any of the five permanent members of the Council (U.S., Russia, Britain, France, and China). Moreover, the U.N. Charter does not simply authorize the Security Council to respond to “aggression.” It also authorizes the Council to act against “breaches of the peace” and “threats to the peace,” which implicitly acknowledges that threats to a nation’s security go beyond an actual cross-border “armed attack.”50
Prior to 1945, international law recognized that nations could respond to lesser threats with measures short of all-out war. These measures might run from mere diplomatic protest to various coercive actions.51 The U.N. Charter also recognizes this tool of traditional statecraft. It allows the Security Council to authorize “measures not involving the use of armed force” against threatening states. Such tactics include “complete or partial interruption of economic relations and of rail, sea, air, postal, telegraphic, radio . . . communications.”52 The Charter also provides that the Security Council may deploy “air-force contingents for combined international enforcement action” when “urgent military measures” are required.53