It sent its bullet through the strongest breastplate, and was therefore decisive against the heavy cavalry, which got into disorder as soon as the men began falling. Ten or 15 musketeers were placed with every company of pikernen, and the effect of their fire at Pavia astonished both allies and enemies. Frundsberg relates that in that battle a single shot from such a musket would bring down several men and horses. From that time dates the superiority of the Spanish infantry, which lasted for above 100 years. - The war consequent upon the rebellion of the Netherlands was of great influence on the formation of armies. Both Spaniards and Dutch improved all arms considerably. Hitherto, in the armies of mercenaries, every man offering for enlistment had to come fully equipped, armed, and acquainted with the use of his arms. But in this long war, carried on during 40 years on a small extent of country, the available recruits of this class soon became scarce. The Dutch had to put up with such able-bodied volunteers as they could get, and the government was now under the necessity of seeing them drilled. Maurice of Nassau composed the first drill regulations of modern times, and thereby laid the foundation for the uniform instruction of a whole army.

The infantry began again to march in step; it gained much in homogeneity and solidity. It was now formed into smaller bodies; the companies, hitherto 400 to 500, were reduced to 150 and 200 men, 10 companies forming a regiment. The improved musket gained ground upon the pike; one third of the whole infantry consisted of musketeers, mixed in each company with the pikernen. These latter, being required for hand-to-hand fight only, retained their helmet, breastplate, and steel gauntlets; the musketeers threw away all defensive armor. The formation was generally two deep for the pikernen, and from five to eight deep for the musketeers; as soon as the first rank had fired, it retired to load again. Still greater changes took place in cavalry, and here, too, Maurice of Nassau took the lead. In the impossibility of forming a heavy cavalry of men-at-arms, he organized a body of light horse recruited in Germany, armed them with a helmet, cuirass, brassaris for the arms, steel gauntlets, and long boots; and as with the lance they would not have been a match for the heavy-armed Spanish cavalry, he gave them a sword and long pistols.

This new class of horsemen, approaching our modern cuirassiers, soon proved superior to the far less numerous and less movable Spanish men-at-arms, whose horses they shot down before the slow mass broke in upon them. Maurice of Nassau had his cuirassiers drilled as well as his infantry; he so far succeeded, that he could venture to execute in battle changes of front and other evolutions with large or small bodies of them. Alva, too, had found the necessity of improving his light horse; hitherto they had been fit for skirmishing and single combat only, but under his direction they soon learned to charge in a body, like the heavy cavalry. The formation of cavalry remained still five to eight deep. About this time Henry IV. of France introduced a new kind of mounted service, the dragoons, originally infantry mounted on horses for quicker locomotion only; but in a very few years after their introduction they were used as cavalry as well, and equipped for this double service. They had neither defensive armor nor high boots, but a cavalry sword, and sometimes a lance; they also carried the infantry musket, or a shorter carbine.

These troops did not, however, come up to the expectations which had led to their formation; they soon became a portion of the regular cavalry, and ceased to fight as infantry. In artillery the French maintained the superiority they had gained. The prolonge was invented by them about this time, and case shot introduced by Henry IV. The Spanish and Dutch, too, lightened and simplified their artillery, but still it remained a clumsy concern, and light, movable pieces of effective calibre and range were yet unknown. - With the 30 years' war opens the period of Gustavus Adolphus, the great military reformer of the 17th century. His infantry regiments were composed of two thirds musketeers and one third pikemen. Some regiments consisted of musketeers alone. The muskets were so much lightened that the rest for firing them became unnecessary. He also introduced paper cartridges, by which loading was much facilitated. The deep formation was done away with; his pikemen stood six, his musketeers only three deep. These latter were drilled in firing by platoons and ranks. The unwieldy regiments of 2,000 or 3,000 men were reduced to 1,300 or 1,400, in eight companies, and two regiments formed into a brigade.

With this formation he defeated the deep masses of his opponents, often disposed, like a column or full square, 30 deep, upon which his artillery played with terrible effect. The cavalry was reorganized upon similar principles. The men-at-arms were completely done away with. The cuirassiers lost the brassarts and some other useless pieces of defensive armor; they were thus made considerably lighter and more movable. His dragoons fought nearly always as cavalry. Both cuirassiers and dragoons were formed only three deep, and had strict orders not to lose time with firing, but to charge at once sword in hand. They were divided into squadrons of 125 men. The artillery was improved by the addition of light guns. The leather guns of Gustavus Adolphus are celebrated, but were not long retained. They were replaced by cast-iron four-pounders so light that they could be drawn by two horses: they could be fired six times while a musketeer fired twice; two of these were attached to every regiment of infantry. Thus, the division of light and heavy field artillery was established; the light guns accompanied the infantry, while the heavy ones remained in reserve, or took up a position for the whole of the battle. The armies of this time begin to show the increasing preponderance of infantry over cavalry.