The Eighty Years War (1567-1659) has been the subject of important monographs but the high command of the Army of Flanders, which played a decisive role in the making of Spanish strategy and was in charge of its tactics, has eluded detailed scrutiny. This work, the first study of an early modern officer corps, examines the culture, class structure, and combat effectiveness of the largest army of its day. Combining approaches and insights from social, cultural and military history, it traces the evolution of the leading cadres of the legendary tercios in relation to major trends such as aristocratization and military modernization while revising recent perspectives on Spain’s war against the Dutch and the French in the Low Countries.