This unique booklet by one of the most renowned biographers of the 20th century, Emil Ludwig, was published for the first time in Spanish in 1939, in a translation prepared by Francisco Ayala in the Buenos Aires in which he had gone into exile. For his work, Ayala drew on the author's original manuscripts as he was writing them. Today we recover this book, in which the reader will find the analytical capacity of a biographer who, already at an early date, is capable of studying the mechanisms and motivations of certain behaviors that are contemporary with him. Ludwig had met with Mussolini and Stalin, and drew their portraits from life for them. The same did not happen with Hitler, whom he described without having met him. Of particular interest is the last chapter, in which Ludwig looks for the origin of German militarism in the Prussian spirit.