The river of history has many branches. Sometimes all it takes to change its course is a chance encounter— which reverberates across the ages.

Insidious intrigues, complicated conspiracies, and ingenious maneuvers: Momentous events and indomitable personalities often shape the course of human history. And yet the devil is in the details: Sometimes a tiny coincidence can set an unplanned chain of

Could the flapping of a butterfl y’s wings in Brazil give rise to a tornado in Texas? This is the question that had piqued the avid curiosity of the American meteorologist and father of the chaos theory Edward Norton Lorenz—and prompted his discovery of the so- called butterfl y effect. His thesis: Given a world in which anything is possible, it can’t be ruled out that the flapping of a butterfly’s wings could give rise to an environmental catastrophe a few thousand miles away. Drawing on meteorological training from MIT, Lorenz had the entire global weather system in mind as he pondered the matter. And in such a global context, tiny random actions can have gigantic consequences. But does the butterfly effect apply to other systems as well? Could it have impacted the course of history, given the countless players, decisions, and events involved? Published in 1963, Lorenz’s thoughts on unpredictability have inspired generations of scientists and historians ever since— and produced spectacular findings. Even small events can affect the big picture and have an enormous effect on an outcome. Weather alone has played a signifi cant role in history. The dropping of the atomic bomb over the Japanese city of Nagasaki on August 9, 1945, for example, had come about due to thick cloud cover obscuring the initial target, the smaller industrial city of Kokura. The pilot, who’d been ordered to drop the bomb by sight rather than by using radar, diverted to Nagasaki instead. But what caused cloud cover over Kokura? Could it have arisen from butterfl y wings flapping somewhere in Brazil? The results, in any case, were absolutely devastating: Of the nearly 300,000 people present in Nagasaki at the time of the blast, 70,000 were dead by the end of that year—and most of them would have been alive if the weather had been different. The accounts on the pages that follow illustrate just how unpredictable the butterfly effect can be— and how profoundly it influences history…