In June 1944, an Allied army of British, American, and Canadian troops sought to open up a Second Front in Normandy. But they were not only fighting to bring the Second World War to an end. After decades of Anglo-American struggle for dominance, they were also contending with one another—to determine who would ascend to global hegemony once Hitler's armies fell. Marc Milner traces this bitter rivalry as it emerged after the First World War and evolved during the fragile peace which led to the Second.

The most highly decorated German regimental commander of World War II, Hyazinth Graf Strachwitz first won the Iron Cross in the Great War. He was serving with the 1st Panzer Division when the Polish campaign inaugurated World War II. Strachwitz's exploits as commander of a panzer battalion during the French campaign earned him further decorations before he transferred to the newly formed 16th Panzer Division. There, he participated in the invasion of Yugoslavia and then Operation Barbarossa, where he earned the Knight's Cross.

There was one submarine that outfought all other boats in the Silent Service in World War II: the USS Tang. Captain Richard Hetherington O’Kane commanded the attack submarine that sunk more tonnage, rescued more downed aviators, and successfully completed more surface attacks than any other American submarine. These undersea predators were the first to lead the offensive rebound against the Japanese, but at great cost: Submariners would have six times the mortality rate as the sailors who manned surface ships.

Populist rage, ideological fracture, economic and technological shocks, geopolitical dangers, and an international system studded with catastrophic risk—the early decades of the 21st century may be one of the most revolutionary periods in modern history. But they are not the first. Humans have lived, and thrived, through more than one great realignment. What makes an age a revolutionary one? And how do they end?

August 25, 1943. A lone bicyclist transports a cache of explosives, hidden in a beer crate, to a Copenhagen hall being readied to house German troops. In a violent blast, the would-be barracks is reduced to rubble. It's the boldest act yet of Holger Danske and the growing Danish resistance combating the oppressiveness of Hitler's Reich. In 1940, on its way to conquering Western Europe, Germany coerced the Danish government into a "cooperative" agreement that lasted three long years until the increasing brazenness of the Resistance movement prompted a crackdown.