Warmatrix

The Tercio was an infantry formation that consisted of a mix of pikemen, arquebusiers and musketeers. It was developed by the Spanish in the early 16th century CE. The Tercio was the first 'pike and shot' formation, dominating the battlefields of western Europe for about a century.
After his experiences in the Italian Wars between Spain and France, especially the Battle of Seminara in 1495 CE, general Gonzalo Fernández de Córdoba reorganized the Spanish army, to counter the threat of French heavy cavalry. The Swiss mercenaries had already demonstrated the power of pikemen against cavalry and German Landsknechte had added guns to the pike formation for extra versatility. The Spanish created "colunelas", units of 1,000 men that consisted of pikemen and gunners in about equal numbers, plus a small number of swordsmen. With these, they won the Battle of Cerignola in 1503 CE.
In 1534 CE the colunelas were combined by groups of three into new "tercio" units. On the battlefield, the pikemen formed a defensive hollow square. Gunners were subdivided into smaller "manga" units, deployed at the edges, from where they fired on the enemy. When threatened, they fell back into the square. The swordsmen were dispensed with.
Tercios were easy to command and packed a lot of pressing force and firepower. They had a mix of pikes and guns that was about optimal during the time that they operated. But there were disadvantages too: tercios moved slowly and having different troop types switch places was a difficult maneuver. Once a tercio was committed to advance, it was hard to make it stop. They were not very efficient in the use of all the weapons at their disposal, but had the advantage of being able to fire in all directions. They were often arranged in a staggered formation, so they could shoot at enemies from multiple sides if these got in between two tercios.
Originally there were four 'old' tercio units: Lombardia, Napoli, Sicilia and Sardinia. Their number later grew. The composition of the tercio also changed. In the later 16th century CE the percentage of pikemen fell to 40% and more and more arquebusiers were replaced by musketeers. Around 1630 CE, most units were reduced in size to 1,500 or as few as 1,000 men. The fraction of pikemen was reduced to 30% or just 20%. The proportion of arquebusiers increased again, at the cost of the musketeers. There was more variation in the width and depth of the formation and commanders placed greater emphasis on the flexible mangas.
After its first successes, especially at the Battle of Pavia in 1525 CE, other countries adopted the tercio formation too. But their troops lacked the discipline, experience and professionalism of the Spanish and never attained the same level of effectiveness. The power of the tercios was first challenged at the Battle of Nieuwpoort in 1600 CE, when they were heavily damaged by volley fire from Maurice of Nassau's prototype line infantry. During the 17th century CE their effectiveness gradually declined and they were transformed to regiments. After a resounding defeat at the Battle of Rocroi in 1643 CE most countries dispensed with tercios; only the Spanish held on to them until 1704 CE.