In his new biography of Henry Kissinger, the historian Niall Ferguson recalls that halfway through what became an eight-year research project, he had an epiphany. Tracing the story of how a young man from Nazi Germany became America’s greatest living statesman, he discovered not only the essence of Kissinger’s statecraft, but the missing gene in modern American diplomacy: an understanding of history.

For Ferguson, it was a humbling revelation. As he confesses in the introduction to Kissinger: “In researching the life and times of Henry Kissinger, I have come to realize that my approach was unsubtle. In particular, I had missed the crucial importance in American foreign policy of the history deficit: The fact that key decision-makers know almost nothing not just of other countries’ pasts but also of their own. Worse, they often do not see what is wrong with their ignorance.”

Ferguson’s observation reminded me of an occasion three years ago when, after an absence of four decades, Kissinger returned to Harvard. Asked by a student what someone hoping for a career like his should study, Kissinger answered: “history and philosophy”—two subjects notable for their absence in most American schools of public policy.

How did Kissinger prepare for his first major job in the U.S. government as national security advisor to President Richard Nixon? In his words, “When I entered office, I brought with me a philosophy formed by two decades of the study of history.” Ferguson uncovered a fascinating fragment from one of Kissinger’s contemporaries when they were both first-year graduate students at Harvard. John Stoessinger recalled Kissinger arguing “forcefully for the abiding importance of history.” In these conversations, Stoessinger said, Kissinger would cite the assertion by the ancient Greek historian Thucydides that “The present, while never repeating the past exactly, must inevitably resemble it. Hence, so must the future.”

“More than ever,” Kissinger urged, “one should study history in order to see why nations and men succeeded and why they failed.”

Ferguson has crafted his biography of Kissinger not only as the definitive account of an incredible personal and intellectual odyssey, but also as an opportunity to initiate a debate about the importance of history in statecraft. The book plants a flag for a project in “Applied History,” which he and I have been gestating at Harvard for several years. By Applied History we mean the explicit attempt to illuminate current policy challenges by analyzing historical precedents and analogues. Following in the footsteps of the 1986 classic Thinking in Time by Ernest May and Richard Neustadt, our goal is to revitalize Applied History both as a discipline in the university and as an art in the practice of statecraft.  

How does Kissinger apply history? Subtly and cautiously, recognizing that its proper application requires both imagination and judgment. As Kissinger put it, “History is not … a cookbook offering pretested recipes. It teaches by analogy, not by maxims.” History “can illuminate the consequences of actions in comparable situations.” But—and here is the key—for it to do so, “each generation must discover for itself what situations are in fact comparable.”

Ferguson’s biography offers an array of examples of when Kissinger drew comparable analogues from history to illuminate contemporary issues and choices. For clues in coping with the frequently frustrating behavior of French President Charles de Gaulle in the 1960s, Kissinger suggested thinking about German leader Otto von Bismarck. For instance, responding to de Gaulle’s moves toward European confederation and away from American influence, Kissinger noted that the French president’s “diplomacy is in the style of Bismarck, who strove ruthlessly to achieve what he considered Prussia’s rightful place, but who then tried to preserve the new equilibrium through prudence, restraint, and moderation.” This insight led Kissinger to conclude that de Gaulle was a self-interested but reasonable leader whom the United States could deal with, at a time when many were ready to write de Gaulle off as a communist sympathizer for being the first Western leader to recognize Maoist China in 1964.

In the 1950s, when mainstream conservatives were ambivalent about Senator Joseph McCarthy’s broadside against alleged communist sympathizers in the State Department and across American society, Kissinger sought to remind them of the complacency of Germans during Adolf Hitler’s early years. As he wrote, “It took some of the best elements in Germany six years after Hitler came to power to realize that a criminal was running their country which they had been so proud of considering a moral state.” The challenge was “to convince the conservative element that true conservatism at the moment requires … opposition to McCarthy.” Using an early version of what Applied Historians might recognize as the “May Method,” in 1951 Kissinger wrote to the CIA’s leading theorist of psychological warfare to set out the similarities and, just as importantly, the differences between 1951, when the United States, the Soviet Union, and Western Europe were struggling to stabilize global order amid the Cold War, and 1815, when European nations constructed an enduring balance of power at the Congress of Vienna.

In reasoning from history, Ferguson explains, the “counterfactual–what might be and might have been—is always alive in the mind of Kissinger’s statesman. The peace he achieves is always by definition a disaster that has been averted.” Ferguson illustrates this point with a string of counterfactual examples in Kissinger’s writings—none more vivid than the West’s response to Hitler: “If the democracies had moved against Hitler in 1936, for instance, ‘we wouldn’t know today whether Hitler was a misunderstood nationalist or whether he was in fact a maniac. The democracies learned that he was in fact a maniac. They had certainty but they had to pay for that with a few million lives.’”

Ferguson calls this concept the “problem of conjecture”: acting before one is certain to avoid potential but uncertain consequences. This is the challenge policymakers face constantly—whether dealing with Vladimir Putin or the threat of nuclear terrorism from ISIS or al-Qaeda. What price are we willing to pay for greater certainty of an adversary’s intentions and capabilities?  In the case of terrorist groups, if we don’t defeat them today, in their incipient phases, we risk allowing them to mature to the point where they can conduct Paris-style attacks—or even another 9/11—tomorrow.

Central to Kissinger’s statecraft, Ferguson’s masterful biography argues, was his ability to bring a deep knowledge of history to bear on the policy questions he confronted. In doing so, Kissinger demonstrated, as Winston Churchill observed, that “the longer you can look back, the farther you can look forward.”