One hundred years ago today, long lines of British infantry climbed out of their trenches in the Somme region of France and hurled themselves at the entrenched Germans. The next 24 hours would turn out to be the bloodiest day in British military history, with 60,000 casualties. Battalions lost 90 percent of their personnel in an hour. One division, the 36th, took 5,000 casualties in 2 days (56 percent) and had to be pulled out — after one year of training, it had lasted 48 hours in combat. The offensive petered out after three months, having advanced only five miles at the cost of 350,000 British and 70,000 Dominion casualties. For the high casualties and lack of success, historians and memoirists have harshly criticized British generals — “donkeys,” in Alan Clark’s estimation, and “arrogant incompetents” in Robert Graves’ semi-autobiography Good-Bye to All That. More broadly, historians have criticized all World War I generals for failing to anticipate the supremacy of the defensive and the trench system that resulted. But is this criticism for lack of foresight fair? As C.V. Wedgewood said:

History is lived forwards, but it is written in retrospect. We know the end before we consider the beginning, and we can never wholly recapture what it was to know the beginning only.

Looking back on World War I, historians can assemble bits of pre-war history to build a narrative predicting the superiority of the defensive and the resulting trench system. However, considered broadly, lessons of battlefield experience before the war were actually ambiguous and indicated a continuation of traditional maneuver warfare, punctuated by periodic sieges of cities and fortresses, as had been the experience in warfare for thousands of years. Here is the pre-World War I history that the generals had available to learn from.

The siege of Richmond and Petersburg at the end of the Civil War (July 1864 to April 1865) is often cited as an example of what the generals of World War I should have expected. The siege positions, extending 35 miles, comprised a continuous line of trenches and fortifications. Union infantry assaults failed, even when supported by surprise or extensive preparation (for example, the Battle of the Crater, July 30, 1864). With its extensive fortifications, powerful artillery, intense infantry fire, and rapid reinforcement of threatened sectors, the siege did indeed look a lot like the western front in World War I. However, this was an exceptional experience during the Civil War. In the west, the war was one of continuous maneuver, punctuated by periodic city sieges (for example, Chattanooga, Vicksburg, Atlanta). In April 1865, when Grant finally outflanked Lee’s entrenched army, the war in the east broke into open maneuver once again.

The Austro-Prussian war of 1866 never devolved into trenches or sieges at all, but instead saw continuous rapid maneuver aided by railroads and telegraph communications, as had been seen in the American Civil War. The decisive battle, Koniggratz, was positively Napoleonic in its movement of corps over large spaces and in the decisive strike to an exposed flank. The war between two great powers was over in seven weeks.

The Franco-Prussian War (1870 to 1871) looked the same, with mass armies sweeping across the map until the Prussians finally encircled the French at Sedan and Metz. The final siege of Paris, with its concentric fortification lines, artillery attacks, and prolonged operations, looked much like city sieges seen through millennia of warfare. The same could be said of the Russo-Turkish War of 1877 to 1878.

The Russo-Japanese War continued the pattern in 1904. Armies maneuvered over vast distances. The great siege of Port Arthur looked much like the siege of Sevastopol 50 years earlier, evincing the same Russian toughness and engineering prowess and the same unsuccessful result (for the Russians). The machine gun and quick-firing field artillery made their first extensive battlefield appearances and ensured that frontal assaults would be as difficult and costly as before, but they did not fundamentally change the nature of the war.

Immediately before World War I, the Balkan states fought two wars between 1910 and 1913, which, because of their nearness in time and the involvement of major powers, seemed to be reasonable models for forecasting the course of a war in 1914. The byzantine political details need not concern us here, only the conduct of the two wars. Operations in the wars followed the established pattern: mass armies, wide maneuver, a major, successful siege (Adrianople), and short duration — eight months for the first war, seven weeks for the second.

So if all these previous conflicts had exhibited extensive maneuver and come to rapid conclusions, what produced the trench system and stalemate of the western front? The short answer is that the wider distribution of machine guns and quick-firing artillery, the large armies, and especially the relatively narrow front all combined to create a unique set of circumstances. It is worth noting that, while stalemate and trenches were the experience on the western front, maneuver was the experience on the (less-studied) eastern front.

Does this then excuse the generals? In part. Expecting clarity about the future is a fool’s errand, suited to historians looking back and not to military planners trying to look forward. As Richard Danzig observes, “Strategic judgments about future environments are often, one might say predictably, wrong.” But the generals are not completely blameless. They can be criticized for failing to adapt as circumstances unfolded. At the Somme, they tried to adapt by employing innovative artillery tactics and massive fire. Their reluctance to acknowledge failure after the first few days, instead continuing the offensive for three months, is difficult to justify.

So how is this relevant a century later? It reminds us how unclear the future really is. Future historians will no doubt look back on the present day and explain how it was obvious events would turn out as they did and how current decision-makers, military and civilian, were blind not to have seen how changes in the world would affect unfolding events. These future historians will have the advantage of seeing how things came out and knowing which parts of history were relevant and which were dead ends. We don’t have that advantage. Yes, it is worth trying to see where present trends are headed. Yes, it is also worthwhile trying to imagine future conflicts and their nature. But we ought to be humble, recognize the limits of foresight, and be ready to adapt to the inevitable surprises that occur.

Mark Cancian (Colonel, USMCR, ret.) is a senior adviser with the CSIS International Security Program. Colonel Cancian spent over three decades in the U.S. Marine Corps, active and reserve, serving as an infantry, artillery, and civil affairs officer and on overseas tours in Vietnam, Desert Storm, and Iraq (twice).