Spanish Tercio pikemen.

A Tercio Español was a powerful Spanish infantry division during the time of Habsburg Spain known for its victories on European battlefields in the early modern period.

The tercio was an administrative unit with command of up to 3,000 soldiers, subdivided originally into 10, later 12 companies, made up of pikemen, swordsmen and arquebusiers or musketeers. These companies were deployed in battle and were further subdivided into units of 30 soldiers. These smaller units could be deployed individually or brought together to form what were sometimes called Spanish squares. These powerful infantry squares were also much used by other European powers, especially the Imperial Army of the Holy Roman Empire.

The care that was taken to maintain a high number of "old soldiers" (veterans) in the units, and their professional training, together with the particular personality imprinted on them by the proud hidalgos of the lower nobility that nurtured them, made the tercios for a century and a half the best infantry in Europe. Moreover, the tercios were the first to efficiently mix pikes and firearms. Tercio companies dominated European battlefields in the sixteenth century and the first half of the 17th century and are seen by historians as a major development of early modern combined arms warfar