Explanations for the Recent Decline in War

As suggested, different kinds of war (and of violence more generally) have declined at different rates and beginning at different times. This implies that different factors account for these noncorrelated phenomena and that a single integrated explanation, desirable in principle, is probably out of reach.Footnote ⁸ Testing the validity of alternative explanations requires a sophisticated research design, but here I can only provide a brief summary.

With respect to great-power war, the single most important driver of its decline over five centuries is the growing severity or lethality of warfare, which increases the costs of war relative to its benefits.Footnote ⁹ In addition, the benefits of war gradually diminished with the consolidation of the modern state system and the gradual settling of territorial borders,Footnote ¹⁰ and later with industrialization and the growth of economic interdependence. These factors diminished the territorial basis of military power, decreased the utility of territorial conquest relative to trade as strategies for the cumulation of wealth, and increased the economic-opportunity costs of war.Footnote ¹¹

Scholars have given more attention to explanations of the “long peace” since World War II.Footnote ¹² The leading factors that Goldstein (pp. 42–43) briefly mentions—changing norms of violence, nuclear weapons, growing prosperity, the democratic peace, and the end of the Cold War—require some elaboration. Norms of behavior (and the broader category of attitudes toward war) unquestionably shifted after the two world wars,Footnote ¹³ and the romantic militarism and social Darwinism of the late nineteenth century have vanished.Footnote ¹⁴ The question is the extent to which shifting attitudes toward war are endogenous to the increasing destructiveness of war, especially with the emergence of nuclear weapons. Autonomous attitudes toward war may help to explain postwar norms against territorial conquestFootnote ¹⁵ and Western (and especially European) hesitancy to intervene in ongoing conflicts, but they have had less causal impact on superpower behavior and on regional conflicts.

The conventional wisdom is that nuclear weapons played a stabilizing role during the superpower rivalry of the Cold War period. They reinforced deterrence by enormously increasing the costs of war and reducing any uncertainty about those costs.Footnote ¹⁶ Although some have questioned the stabilizing effects of nuclear weapons,Footnote ¹⁷ the majority who accept the idea of a nuclear peace between nuclear powers are more confident than those who do not that future Sino-American disputes—over Taiwan, the South China Sea, or other issues—will be resolved peacefully. Of course, inadvertent wars occasionally occur,Footnote ¹⁸ and there is no reason to believe that the psychological biases driving threat perception and decision making have diminished in any way,Footnote ¹⁹ but psychological biases must overcome much greater structural disincentives than they faced in the past in order to lead to war.

Whether further nuclear proliferation would be stabilizing is a different question, one that has generated considerable debate.Footnote ²⁰ Critics emphasize crisis instability between states with limited weapons and insecure second-strike capabilities, which can generate temptations for preemption. In addition, fears that an adversary might soon acquire nuclear weapons capability increase incentives for a preventive strike to eliminate or delay that possibility.Footnote ²¹

Goldstein mentions prosperity as a source of peace. Growing prosperity undoubtedly helps to explain the decline of civil wars,Footnote ²² although the organizational strength of the state is an equally important factor.Footnote ²³ The idea of economic forces pushing toward peace among advanced industrial states is usually framed in terms of economic interdependence rather than prosperity, and in fact the “capitalist peace” now rivals the “democratic peace” as primary explanations for the near absence of wars between democracies in the last two centuries.Footnote ²⁴ Both contribute to peace in the West, but neither can explain peace between the superpowers during the Cold War.

Finally, Goldstein invokes the end of the Cold War as an explanation for the decline in civil war since 1989, emphasizing the withdrawal of external financial support for rebels.Footnote ²⁵ Many go further and argue that U.S. hegemony has helped to maintain order and minimize the numbers of wars, and that the decline of U.S. dominance will increase the likelihood of conflict.Footnote ²⁶ They often refer to power-transition theory, long-cycle theory, and theories of unipolarity, which argue that high concentrations of power significantly reduce the likelihood of great-power war by eliminating hegemonic rivalry and counterhegemonic balancing.Footnote ²⁷ However, the theoretical linkages between unipolarity and other forms of warfare (major–minor, minor–minor, and internal) are less well developed, and one can find lengthy periods of relative peace in multipolar worlds.Footnote ²⁸ Hegemonic theories have important but conditional implications for the consequences of a possible Sino-American rivalry for dominance, but no clear implications for other forms of warfare.

The Hazards of Forecasting

Goldstein emphasizes that the continued decline in war is not inevitable (pp. x, 6, 42), but he is cautiously optimistic about the future. I agree with him that the likelihood of a major–major war is quite small (though nontrivial in the case of a Sino-American war), but I do not share his optimism about other types of war. The combination of resource scarcities (especially food and water), droughts and other climatological shocks, and ensuing migrations, along with certain demographic trends (the anticipated increase in potentially disaffected young male populations in sub-Saharan Africa and parts of the Middle East),Footnote ²⁹ and the lack of congruence between state and national boundariesFootnote ³⁰ could create conditions conducive to civil wars and their possible internationalization. Numerous flash points, in the Middle East and elsewhere, make it unlikely that the relative interstate peace of the last decade will persist. However, rather than defend these speculative remarks, I want to emphasize the hazards of forecasting.Footnote ³¹

Imagine a Perspectives on Politics symposium on the decline of war taking place a hundred years before this writing, in early November 1912. The dialogue would have focused on Norman Angell's The Great Illusion (Reference Angell1910) and perhaps on Ivan Bloch's book subtitled Is War Now Impossible? (Reference Bloch1899). Each argued that a war between the leading industrial powers would be long, economically devastating, socially disruptive, and consequently irrational. Pessimists might have noted that a war had begun in the Balkans a few weeks before, but no one expected the major powers to intervene, and in fact they did not. Contributors would have noted that they were living in one of the most peaceful periods in modern history. The “long peace” between the major powers of Europe had persisted for more than four decades, the longest such period in four centuries. A hegemonic war involving nearly all of the major powers had not occurred for nearly a century. In addition, the average duration of great-power war and median number of battle deaths in wars continued to decline. The four great-power wars since the Congress of Vienna had each lasted less than a year on average, reflecting a significant decline from the wars of previous centuries. There had only been one European war in the previous three decades (the Greco-Turkish Thirty Days' War of 1897),Footnote ³² and the frequency of civil wars had declined by half over the previous four decades.

True, there had been frequent crises involving the major powers—over Morroco in 1905, Bosnia-Herzegovina in 1908, and Agadir in 1911—but each crisis had been resolved peacefully, enhancing confidence in the effectiveness of crisis management. True, some military leaders had advocated a preventive war, but those pleas had been rejected by statesmen like Otto von Bismarck. No European great power had incorporated preventive war into its national security strategy or publically used preventive logic to justify military action.

There were other grounds for optimism. A détente continued between the two leading European powers, Great Britain and Germany, enhanced by the strong commercial and financial relationships between the two countries. The historically unprecedented levels of economic interdependence further reinforced the peace, based on the increasingly popular arguments of Norman Angell, Manchester liberals, and others that wealth was based on credit and commerce, and that territorial conquest was no longer an efficient strategy for increasing wealth. The leading powers had too much at stake to go to war.

For all these reasons, the contributors to a 1912 Perspectives symposium would have had strong grounds on which to forecast a continuation of the long peace. In fact, in many respects, the quantitative trends pointing in that direction were stronger than those emphasized by the “declinists” of 2012. There was a more sustained decline in great-power war and a longer period without a hegemonic war. The occurrence within two years of what George Kennan called the “the great seminal catastrophe” of the twentieth century,Footnote ³³ resulting in more than eight million battle fatalities, serves as a cautionary tale about the limitations of forecasting.Footnote ³⁴