When asked about the value of studying history, most students respond with some version of the Spanish-born philosopher George Santayana’s famous dictum: “Those who cannot remember the past are condemned to repeat it.” At one level, this notion of learning from the “lessons of history” has much to recommend it, for there is, after all, little else except the past on which we can base our actions in the present. And yet historians in general are notably cautious about drawing particular lessons from the past and applying them to present circumstances.

For one thing, the historical record is sufficiently rich and complex to allow many people to draw quite different lessons from it. The world wars of the twentieth century represent a case in point, as writer Adam Gopnik has pointed out:

The First World War teaches that territorial compromise is better than full-scale war, that an “honor-bound” allegiance of the great powers to small nations is a recipe for mass killing, and that it is crazy to let the blind mechanism of armies and alliances trump common sense. The Second teaches that searching for an accommodation with tyranny by selling out small nations only encourages the tyrant, that refusing to fight now leads to a worse fight later on…. The First teaches us never to rush into a fight, the Second never to back down from a bully.19

Did the lessons of the First World War lead Americans to ignore the rise of fascism until the country was directly threatened by Japanese attack? Did the lessons of World War II contribute to unnecessary wars in Vietnam and more recently in Iraq? There are no easy answers to such questions, for the lessons of history are many, varied, and changing.

Behind any such lesson is the common assumption that history repeats itself. This too is a notion to which historians bring considerable skepticism. They are generally more impressed with the complexity and particularity of major events such as wars than with their common features. Here is a further basis for caution in easily drawing lessons from the past.

But the wars of the past century perhaps share one broad similarity: all of them led to unexpected consequences. Few people expected the duration and carnage of World War I. The Holocaust was unimaginable when Hitler took power in 1933 or even at the outbreak of the Second World War in 1939. Who would have expected an American defeat at the hands of the Vietnamese? And the invasion of Iraq in 2003 generated a long list of surprises for the United States, including the absence of weapons of mass destruction and a prolonged insurgency. History repeats itself most certainly only in its unexpectedness.