The reign of Philip IV of Spain is fascinating, as after a century of dominance in Europe, the Spanish hegemony was seriously challenged by France. At the beginning of Philip IV's reign, Spanish dominions were vast, not only in Europe but also in South America, Asia and Africa. The defense policy of such vast territories was established in the XVI century and consisted of the deployment of two core armies, in the Low Countries and in north Italy, the maintenance of strategic garrisons, and of fleets in the Atlantic and the Mediterranean to control communications between all the territories of the monarchy. The different military formations were actively supported by a powerful diplomatic network among the European countries.

The aim of the book is to examine the armies and troops of Philip IV and particularly the famous Spanish Tercios deployed in north Europe, Italy and in the Iberian Peninsula. The book covers the organization and development of the Tercios, the tactics used, recruitment and the life of the ordinary soldiers. The development of the Spanish cavalry and its organization is also covered in depth, along with the financing of the army.

The book also explores the impact of continuous fighting during a period of 44 years to keep the honor and reputation of the Spanish monarchy, as well as their territories intact, and how the army responded to these challenges.