Throughout these pages, you will discover how American, Russian and German spies, among others, infiltrated the lines of both sides, wove intelligence networks and carried out missions of vital importance, both for their national interests and for the development of the Spanish conflict. These men and women played a decisive role, from gathering information to making strategic decisions that would influence the course of events. This book offers a detailed look at the intricate details of the espionage operations that not only marked the Spanish Civil War, but also left a palpable legacy in World War II. From the dark alleys of Madrid to the upper echelons of international politics, a multitude of clandestine operations took place on the scene of a devastating conflict, and Spain became a kind of spy school for the future confrontations that would take place in the 20th century. An essential work to learn more about the characters and events that changed the course of nations and the impact of intelligence operations on the global scenario of that convulsive historical period.