During the first week of August, Ukraine launched an incursion into Russia’s Kursk region in what constituted the largest foreign attack on Russian territory since Germany invaded the Soviet Union during World War II. In a 2017 essay, the historian and Stalin biographer Stephen Kotkin told the story of that earlier assault through the eyes of two of its chief actors: Joseph Stalin and Adolf Hitler.

With Stalin coming to power in 1924 and Hitler in 1933, the lives of the two dictators had somewhat “run in parallel,” Kotkin observed. “But it was the intersection that would matter: two very different men from the peripheries of their societies who were bloodily reviving and remaking their countries, all while unknowingly (and then knowingly) drawing ever closer.” Two years into World War II, miscalculations on both their parts would set off a chain of events leading to one of the largest—and deadliest—military confrontations of all time. “History is driven by the interaction of geopolitics, institutions, and ideas,” wrote Kotkin, “but it takes historical agents to set it all in motion.”

In the summer of 1941, it seemed that Hitler was winning the war, as Stalin continued to observe the nonaggression pact that Germany and the Soviet Union had signed in August 1939. For months, though, rumors had spread about a possible German invasion of the Soviet Union, raising fears about an “imminent titanic war.” But Stalin, Kotkin wrote, was “driven by a blinding desire to avoid war at all costs” and branded as disinformation “whatever he chose not to believe.”

Thus, when over three and a half million German and Axis troops attacked the Soviet Union on June 22, 1941, in Operation Barbarossa, Stalin was, despite months of warnings, caught off guard. But in the end, Kotkin contended, “the question of who most miscalculated is not a simple one.” Anticipating triumph over Moscow, Hitler had opened up the eastern front of the war—a theater that would ultimately prove fatal for German forces. The Soviet Union would triumph over Germany, but at a terrible and lasting cost.