Ace or anybody else, you've mentioned Tercio units a few times. From what I can understand, they were large blocks of pikemen with smaller groups of arqubuisers at each corner. Is that correct?
How did this work in practice when they were charged by cavalry? Did the gunners move within the block of pikemen?
As Furius stated, suggestions that the gunners mingled in with the pikes makes no sense. The Tercio formation involved a complex, yet disciplined and effective system of cycle firing. A formation of well-trained gunners (the Spanish were masters of the system with the Dutch being probably next in competency) could cycle through a formation quickly. With 2 minutes to discharge fire, it seems very likely to me that they could accomplish two things. Firstly, they could cause casualties. This is always useful in battle. Secondly, they could cause a great deal of smoke from the guns. This would confuse enemy cavalrymen. Thirdly, at a close enough range, these discharges of lines of gunners firing almost simultaneously would undoubtably have created a huge noice that likely would've made it difficult for horsemen to charge straight in. You also have to consider that the range would be greater. The arquebusiers that would've been the normal gunners would've been supplemented by musketeers at the extremities of the formation.
Against infantry, however, the gunners would likely have taken a more of a skirmishing and preventive stance. I think that this is what the pikemen are mainly for. Against cavalry, it seems logical that since horse will not collide with a solid formation, even if the corner of gunners does break, they'll likely try to retreat into the pikes, if anywhere, where it would be impossible for the horse to follow. In that way, there's no chance of the horse truly driving away the gunners.
In any case, back to infantry. It seems that the gunners, not the pikemen, would have been the main deterent against cavalry. The melee infantry in the center seems to be an anti-infantry measure. According to a book I read on the subject, they described a "forlorn hope" of musketeers having been deployed in front of the pikemen and further up from the formation as a skirmishing force against enemy gunners. I think that all of the gunners would've served mainly as skirmishing forces in the time when the formations of melee infantry began closing. After this time, I would conjecture that the gunners would retreat to the flanks where they could fire away at any flanking threats, infantry or cavalry, as well as providing nominal covering fire for the infantry against the enemy's infantry. There it would be the professionalism of the Tercio's pikemen and halberdiers that would be expected to win the infantry engagement, as often happened. Naturally, such a system calls for very deep formations, which are huge targets on a battlefield. This meant that when cannons became truly effective on the battlefield, it spelled the death of the Tercio as the formation could just be blasted to pieces by artillery bombardments. Even as such, in the final "hurrah" of the Spanish tercio at Rocroi, the Eastern Habsburg and mercenary formations broke while the Spanish tercio actually managed to withstand about four French cavalry charges with heavy cannonades in between. To illustrate best what kind of formation the Tercio could be, the surrender terms that were offered to the Spanish were the same terms that would have been offered to a force besieged inside of a fortification. The remaining men that surrendered were allowed to leave the field with their weapons and their banners raised. Of course, the battle was a total diaster for the tercio. The Spanish formations broke every French cavalry charge, but they suffered massive casualties from cannon fire since the very thing that allowed them to repel the attacks (the dense formations) made them sustain abusive casualties from the artillery.
Other notes about it are that it was only comprised of men who volunteered to serve specifically in the tercio formations. They were recruited from anywhere and could even included men from countries that were enemies of the Habsburgs at the time. Of course, the men were mainly Hispanic or Germanic, the two largest groups from Habsburg lands or Italian, most of whom were mercenaries or from Italian Habsburg lands (The Kingdom of Two Sicilies). A special royal liscence was also needed to be able to recruit men for the tercio.
Of course, the 1:1 correspondence between pikes and guns was only the origional one. By the time that the Spanish were operating in the Netherlands, the ratio heavily favored pikemen.
I'm also in the process of reading through Spanish sources online about them. It's actually amazing how vastly greater the amount of information is on the tercio from Spanish sources, while English sources say little about one of the most significant formations in military history. The Spanish Wikipedia article claims that this is partly due to the Black Legend, which prejudicies the formation. While that's probably a part of it, it likely has more to do with the fact that a formation invented by one group likely has the most sources about it in their own language. In any case, I'm going to do some scouring around the internet for information about the tercio from Spanish sites and I'll try to have more solid conclusions about the formation drawn from this meager research.
I put a dollar in one of those change machines. Nothing changed. ~George Carlin[This message has been edited by Ace Cataphract (edited 01-25-2007 @ 00:09 AM).]