Tercio (Sp.: tercio, a third) was a fighting formation developed by González de Córdoba to exploit firearms protected by pikemen, although only given the name in 1534, perhaps to contrast with the similar ‘legions’ of François I. It became a byword for Spanish military power. Unlike earlier formations, the tercio only comprised pikemen and arquebusiers, with none of the ‘sword and buckler men’ whom Machiavelli had so admired. A development of the medieval division of an army into three parts—the van, the main battle, and the rear—the new ‘third’ was a regiment or brigade equivalent, 3, 096 strong on paper and commanded by a maestre de campo—a colonel—part of a staff of nineteen officers controlling twelve companies. At the time it represented a step forward in battlefield flexibility, but it was gradually overtaken by the tactical innovations introduced by Maurice of Nassau and others who followed his example. Its demise as a formation is generally considered to have come with the destruction of the Spanish army at Rocroi in 1643 by a combined arms assault.
— Christopher Bellamy
