Fixed nitrogen is essential in agriculture. Its rarity, as science writer Hager writes, shaped the world and its politics. Hager details that in 1905 German chemist Fritz Haber discovered a process for transforming abundant air-borne nitrogen into ammonia, and Carl Bosch's engineering scaled Haber's benchtop chemistry into industrial processes to make fertilizer. Haber and Bosch earned Nobel Prizes and saved millions from starvation. By 1944, the Haber-Bosch factory at Leuna — a primary target for U.S. bombers — occupied three square miles and employed 35,000 workers. Hager not only illuminates the men's complex work, but also digs into their personal lives. Haber, a Jew, developed the chlorine gas used in World War I, sought a way to extract gold from the oceans to pay off German war reparations, and conducted research that led to the development of the Zyklon B gas used in Nazi death camps. Bosch asked Hitler to spare Jewish scientists for the sake of German chemistry and physics. The Fuhrer replied: "Then we'll just have to work 100 years without physics and chemistry."