by Jack S. Levy

International relations scholars have known for years that the last six or seven decades have been the longest period of peace between the great powers in the last five centuries of the modern system, and, depending upon whom and what you count, perhaps since the Roman Empire. Extrapolating from these and other indicators, many argue that this long peace is highly likely to persist for many years into the future. Although I am less confident that other forms of warfare will continue to decline during the next two decades, and although I suspect that peoples in many parts of the world might be puzzled by the description of the period as a “long peace,” I basically share the optimism about the low likelihood of a future great power war. My optimism is tempered, however, by the recognition that political forecasting is a hazardous business, a point that I develop in these comments.

It would be instructive to imagine the dialogue on an earlier blog on the decline of war, say 100 years ago, to pick a round number. Bloggers in August 1912 would presumably have talked about some recent books bearing on this question, including Ivan Bloch’s book subtitled Is War Now Impossible?(1899) and Norman Angell’s The Great Illusion (1910). Each argued that a war between the leading industrial powers would be long, economically devastating, and socially disruptive, and consequently not rational. The strong inference was that a future great power war was unlikely because – reflecting the logic of the Global Trends 2030 report (iv) a hundred years later – “too much would be at stake.”

Our bloggers might have gone beyond these theoretical arguments to note existing trends in war and to project those trends into the future. Looking back, they had lived through the most peaceful century on record. Perhaps using words similar to those in Allan Dafoe’s introductory comments on this blog, they may have written that “There are fewer great power wars, fewer wars in Western Europe, fewer years during which a great power war is ongoing, and less redistribution of territory after wars.” A long peace between the European great powers had persisted for over four decades, the longest such period in four centuries. A hegemonic war involving nearly all the great powers had not occurred for nearly a century. But it was not just the frequency of great power war that had been in decline. The median number of battle deaths in wars continued to decline, as did the average duration of great power war. 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. Only one European interstate war had occurred in the last three decades (the Greco-Turkish War of 1897), and it only lasted thirty days. The frequency of civil wars had declined by half over the last four decades.

The bloggers of 1912 may have emphasized that although militarized disputes between the great powers continued to occur – over Morocco in 1905, Bosnia-Herzegovina in 1908, and Agadir in 1911 – each had been resolved peacefully. This increased political leaders’ confidence that they would also be able to peacefully resolve any dispute that might arise in the future. True, some military leaders had advocated a preventive war, but those pleas had been rejected by statesmen like Bismarck. No European great power had incorporated preventive war into its national security strategy or publically used preventive logic to justify military action.

The statesmen of 1912 had other grounds for optimism. A détente continued between Great Britain and Germany, the two leading European powers in the system. That détente was motivated in part by the strong commercial and financial relationships between the two countries. In fact, Europe as a whole benefited from historically unprecedented levels of economic interdependence. This further reinforced the peace, based on the increasingly popular arguments of Norman Angell and others, that wealth was based on credit and commerce, and that territorial conquest was no longer an efficient strategy for increasing wealth.

Thus the bloggers of 1912 had reasonable grounds for forecasting a continuation of the long peace.  In fact, in many respects the quantitative trends pointing in that direction were stronger than those observed by bloggers a century later. Compared to the bloggers of 2012, the bloggers of 1912 could point to a more sustained decline in great power war and a longer period without a destructive hegemonic war.

I believe and certainly hope that the parallels between 1912 and 2012 end there. But the occurrence within two years of the Great War – which resulted in over eight million battle fatalities and which is often described as “the seminal event of the 20th century,” should be a constant reminder of the limitations of both trend-based and theory-based forecasts of the future.

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Jack S. Levy is Board of Governors’ Professor of Political Science at Rutgers University. He is past president of the International Studies Association and of the Peace Science Society. His most recent books include Causes of War (2010) and The Arc of War: Origins, Escalation, and Transformation (2011), each co-authored with William R. Thompson. He is co-editor (with Leonie Huddy and David O. Sears) of the second edition of the Oxford Handbook of Political Psychology (forthcoming, 2013), and he is organizing an edited volume on the First World War.