What is the tericco?

The tercio was a Spanish infantry formation that was highly effective during the Early Modern era of “pike and shot,” and a big part of the reason why the Spanish were so dominant in European warfare in this period.

In essence, the tercio was a tightly-packed, well-drilled square formation of mixed pikemen, musketeers, and swordsmen, designed to be extremely durable and very hard to be broken by cavalry. The pikemen would protect the musketeers from cavalry, and the musketeers would in turn direct their firepower at opposing pike, while the swordsmen would be used to attack weakened formations or to fill any gaps in the line. On the battlefield, the Spanish would field multiple tercios in a kind of checkerboard fashion that allowed the tercios to support one another with enfilade fire against enemy units, making them incredibly daunting to assault in hand-to-hand combat. 

(credit to Milgesch)

But more than just a clever formation and use of mixed arms, the Spanish tercio was dominant because Spanish soldiers were experienced professionals with incredible discipline, executing the complex drill of the tercio (where lines of pike and musketeers had to move past one another repeatedly) in the most difficult of circumstances, and keeping their lines together even after absorbing hideous amounts of casualties, which often allowed them to exhaust their opponents.

Gradually, the tercio became obsolete as other strategies evolved to deal with the dominant Spanish. Because of how close-packed and thus slow-moving the tercios were, they were incredibly vulnerable to massed firepower, both from infantry and artillery. For example, Maurice, Prince of Orange moved his armies into longer, thinner lines that could bring more guns to bear on the enemy than square blocks where the sides and rear couldn’t always fire, and found success at the Battle of Nieuwpoort. Likewise, the Duc d’Enghien at the Battle of Rocroi used a combination of superior cavalry to encircle the tercio and massed artillery to blast them to pieces.