For a battlefield formation, the Spanish refined older concepts of infantry formations and instituted the tercio, and the concept spread throughout Western Europe with the building of the Habsburg empire. Spanish organized their companies stationed in Italy into these formations in 1534 and a year later gave them the name ‘tercio.’ Ideally, a tercio numbered 3,000 fighting men, reality revealed smaller formations (due to the exigencies of manpower shortages chronic to the Spanish armies). The tercio’s core was pike, arrayed in deep files, sometimes as many as 30 deep. At the corners of the main block of pike, like bastions on a four-sided fort, were the mangas, four squares of 100 to 400 each of arquebusiers and musketeers. Commanders could have those mangas protect the flanks of the main unit or arrayed in front to lay down fire – or even to go out to skirmish. The tercio provided the opportunity for commanders to make use of even green recruits who could be placed at the backs of the files to at least provide mass, and the recruits learned by doing, and thereby literally advanced up the ranks. Deep formations also made for commitment: once engaged, the front ranks found it nearly impossible to turn and run since the rear ranks would still be pressing in.

The tercio of the 16th century worked well against cavalry and on the attack became a force with great forward momentum. A tercio of coordinated and spirited fighters could hit enemy infantry like a hammer – and drive in to decide the battle. The limitation of the tercio was its very momentum. Once started, a tercio was hard to turn to respond to changing battle conditions. Successful tactics were to place a tercio as well as possible at the outset and then be careful with the timing in launching it. This method was what Tilly had used to win battle after battle until Breitenfeld.

By 1631, the Spanish were reconceptualizing the tercio. The ideal remained 3,000 men in the makeup but with more companies of fewer men per company, and in reality the overall number of men slid to 1,500, even as few as 1,000. The variety of weapons was reduced to the pike, the arquebus and the musket. Pikemen constituted then only about a third of a tercio, the remainder devoted to firepower. Commanders made the tercios more flexible (like adjusting from square to wide and thin) and placed greater emphasis on the mangas. Tilly clung conservatively to the older structure of the tercio as the great block - as has been assumed, but there is a reference that differs. In addition, circumstantial evidence supports that reference.

An English mercenary named Sydnam Poyntz claimed to have served under Mansfeld and Wallenstein, and with the Saxons, then he later saw action as an officer with Parliament in the English Civil War.  Poyntz described the formations in Tilly's army as being only 12 men deep (not 20 or 30 as with the 16th century tercios).  Poyntz' claim fits hand in glove with what Tilly's former commander and mentor, Giorgio Basta, advocated: infantry formations of 12 ranks.  Too, this count matches what the Elector of Saxony mentioned in a letter, that the Imperial-Leaguist army formed up in formations twice as deep as what the Swedes had (which was six deep for musketeers) and - also interesting - twice as deep as the Saxon formations, suggesting the Saxons were following the Swedish example.  That even Tilly could have adopted the thinned formations conforms to the evolution of combat technology, that the firearm came to dominate the infantry, relegating pike to the defensive.  Thus, with the advent of coordinated gunfire, a thick block of pike with its great forward momentum was no longer the deciding factor on the battlefield and fewer ranks could provide adequate defense against horse.

A contributing factor to the Imperialist-Leaguist loss at Breitenfeld was not knowing how quickly and adeptly the new Swedish brigades could adapt to a changing battle compared to their own tercios. The Imperialist-Leaguists learned a hard lesson at this battle – which they applied the following year.