The differentiation in role between pikemen (melee infantry) and arqubusier/musketmen was a facet of warfare going back to the introduction of handheld firearms in 15th Century Europe. This was largely a result of the relatively primitive technology of the time, when early firearms were large, heavy, and cumbersome weapons which, although providing valuable firepower to break up enemy formations, had no way of effectively engaging in melee combat or in driving off a cavalry charge. For this reason, groups of arquebusiers had to be supplemented with formations of pikemen, behind which they could retire when threatened.

The first real standardization of this concept came in the early 16th Century with the development of the Spanish Tercio. Early Spanish Tercios were formed from ten companies of first 300, later reduced to 250, men each. Of these companies, two were exclusively armed with arquebus and eight with a mix of predominantly pikes, supplemented with a smaller number of sword and buckler armed men. This early form of organization for the Tercios was eventually replaced with a more flexible combined-arms companies, each including 150 pikemen, 100 arquebusiers, and 40 sword-and-buckler men. Thus we find that the ratio of early formations used a roughly 1:3 ratio of firearms to melee weapons, which was gradually reduced to a 1:2 ratio as the importance of firepower waxed and the relative importance of cavalry wained.

Later developments would see the number of pikes to firearms continue to fall through the course of the Thirty Years War. The Hapsburgs would continue to use the Tercio, with its 1:2 ratio of arms (albeit without the contingents of sword-and-buckler armed men) during the conflict, while their Protestant enemies, specifically the Sweedes, using thinner formations with a 1:1 ratio of firearms to pike (three ranks of muskets, backed by three ranks of pike). This, combined with developments in drill, volley fire techniques, and the addition of small 3 and 6 pounder cannons to each battalion, would prove a nasty shock to the Tercio formations, which could not bring as much firepower to bear as the thinner Sweedish formations. The Battle of Breitenfield in 1634 decisively proved the superiority of linear formations against the deeper Tercio, and would set the stage for the gradual retirement of the pike and the growth of all-musket armed units.

This transition was complicated by the continuing need for infantry formations to be able to stand up to cavalry, and thus by the available bayonet technology. It is only in the early years of the 17th Century that plug bayonets were adopted, and these had the dual problems of preventing the musket from being loaded or fired while the plug bayonet was attached, and of poorly secured bayonets falling off of the musket to which they were attached. This issue would not be resolved until the beginning of the 18th Century, with the introduction of offset socket bayonets which use spring pressure to hold them securely to the weapon. Incidentally, it is around the time of the widespread adoption of the socket bayonet that most militaries began to finally eliminate the pike from their formations. The French would adopt an effective socket bayonet in 1703. In 1704, the Spanish would officially retire the Tercio (by this point already quite changed from its original inception a century and a half prior) and remove the pike from regular service. Militia units carrying pikes would continue to be fielded on occasion, but were not expected to stand against modern linear formations as the Tercio had.

For more information on the "Military Revolution" caused by firearms, please read John Keegan's "A History of Warfare," or Michael Robert's "Sweden as a Great Power 1611-1697," or one of Robert's other works on Gustavus Adolphus and the Vasas.