The tercio (literally "third") or tercio español (literally "Spanish third") was a Spanish military unit of the Spanish Empire during the era of the dominance of Habsburg Spain in Europe and the Renaissance era.
The tercio was an infantry formation made up of pikemen, swordsmen, and arquebusiers or musketeers in a mutually supportive formation, that in theory was up to 3,000 soldiers, although it was usually less than half this size. It was also sometimes referred to as the Spanish Square in other countries and the formation was also much used by other powers, especially the Imperial Army of the Holy Roman Empire.
Spanish tercios were the first modern European army, understood as an army of professional volunteers, as opposed to the conscripted levies and hired mercenaries typical in other European countries of the time. 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. The formation dominated European battlefields in the sixteenth century and the first half of the seventeenth century and is seen by historians as a major development of Early Modern combined arms warfare.
During the 17th century use of the tercios declined as new tactics were created that could directly challenge them and started to lose major battles that shattered their image as an invincible unit, though tercios continued to achieve victories for the Spanish. In response, the later 17th-century "tercios" adopted so much of the linear style's organization and tactics as to have little resemblance to the classic tercios of the previous century, with the men now armed with the musket. In 1704, the Spanish regular tercios were transformed into regiments, those of the reserves and the militia would later be transformed into similar formations.
A squad of Tercios were walking in a plain field with few trees around, until the officer didn't spot a destroyed village, which is still burning. Not away a boyar girl that managed to escape, she saw them and with tears in her eyes, she pointed at the village, as five Streltsy soldiers emerged. As the girl began to run, the two arquebusiers aimed their guns and fired at the Streltsy and one of them was shot at the abdomen and killed.
The Streltsy aimed their pischals on their bardiches, and fired, killing two of the Tercios. Than both sides charged at each other with their polearms, and the Tercios knelt down with their partisans only managing to impale one unlucky Streltsy in the chest.
While the other three remaining Streltsy swung their bardiches and destroyed all of the Tercio's partisans. As the Tercios began to back away and as one struggled to pull his rapier from the pressure and for this reason, he got his head nearly severed by a single blow of the bardiche. As his companion tried to kill a Tercio with overhead swing, this cost him his life, because the Tercio pierced his throat with his toledo rapier and as the Streltsy began choking blood from his mouth.
The other two Streltsy were coming toward the other two Tercio's, and one of the Russians pulled his sablia and began swinging forward, the first hit bounced from the cuirass and this cost the Russian his life, because the Tercio pierced his heart with the parrying dagger. The last remaining was the captain of the Streltsy and he began swinging his bardiche, and he managed to cut the the leg of the Tercio and as the Spaniard felt to his knees, the Russian pulled his sablia and decapitated him.
As the captains of both the Tercio and Streltsy were holding their swords and both clashed, with the Tercio parrying the Russian's blows. After few unsuccessful swings from the sablia, the Streltsy pulled his kindjal and stabbed the throat of the Spaniard, which sprayed blood onto his face. As the Tercio's body collapsed, the Russian raised his kindjal and yelled in victory.
While the Tercio had better melee weapons and armor, but the Streltsy were more ferocius and had superior polearm and firearm. Possibly the biggest advantage for the Streltsy is that they are suited fro small squad battles, unlike the Tercios who are good at big army battles.
Battle vs. British Army (by Detective Loki)