With the centenary of World War I’s outset approaching, historians and foreign-policy experts are warning leaders to revisit its lessons, lest they allow such catastrophes to repeat themselves. Among those lessons: never underestimate the power of misbegotten ambition. "If we cannot determine how one of the most momentous conflicts in history happened, how can we hope to avoid another such catastrophe in the future?" Margaret MacMillan asked in the New York Times last December. "Instead of muddling along from one crisis to another," she concluded, "now is the time to think again about those dreadful lessons of a century ago—in the hope that our leaders, with our encouragement, will think about how they can work together to build a stable international order." 

We should heed MacMillan’s wisdom. But we should also appreciate the progress that global security has made in the intervening century. 

Granted, it is difficult to assess whether the world is “safe” in an absolute sense—and after reading U.S. Director of National Intelligence James Clapper’s latest threat assessment, it is easy to see why most people, if pressed, would say that it is not. If, however, one considers a very rough metric—the likelihood that calculated actions will kill millions of people in a short span of time—there are grounds for believing that the world is becoming safer. 

The emergence of international order from the horrors of the twentieth century can be difficult to appreciate, especially for those of us who came of age after the Cold War. Some 20 million were killed in World War I, and more than three times that number perished in World War II. The Cold War may have been a “long peace” at the highest level of analysis—a third world war did not occur—but that phrase conceals the toll of proxy wars, civil wars, genocides, and other conflicts. In 1993, Zbigniew Brzezinski estimated that “during the twentieth century, no less than 167,000,000 lives—and quite probably in excess of 175,000,000—were deliberately extinguished through politically motivated carnage.”

Surveying the threats on today’s global agenda, it is easy to envision how many of them could escalate into crises that stretch diplomatic restraint to its limit. It is, thankfully, more difficult to imagine how they could culminate in the destruction of international order. Consider the following four ...