Meanwhile, the improvement of firearms caused armour to be discarded. Infantry ceased wearing it almost completely after 1660, and the armour carried by cavalrymen grew steadily shorter until all that remained were the breastplates worn by heavy cavalry—the cuirassiers—as late as the 20th century. The harquebus developed into the heavier, more powerful musket, which soon acquired the flintlock firing mechanism. This was scarcely the perfect weapon, but it could be relied on to fire two or three times per minute to an effective range of 100–150 yards without misfiring more than 20 percent of the time. There was a constant tendency to increase the number of musketeers at the expense of pikemen until, by the end of the Thirty Years’ War (1618–48), their proportions had become about equal. To allow the maximum number of barrels to fire without mutual interference, tactical units grew smaller, and the number of ranks drawn up behind one another declined. From 8 to 10 at the time of Prince Maurice of Nassau early in the 17th century, it came down to 4 or 5 a century later, 3 or 4 in the armies of Frederick the Great, and 2 or 3 toward the end of the 18th century.

To maximize efficiency, drill was invented. It first made its impact felt in the Dutch army under Maurice of Nassau, a great teacher whose headquarters attracted aspiring officers from all over Europe. Standards, often modeled after Roman ones, were introduced to help units align themselves, and tactical movements were carried out to the sound of trumpets, bugles, and drums—the latter an Oriental innovation apparently brought to Europe about 1500. In this age of René Descartes, Thomas Hobbes, and Louis XIV, each of whom in his different way was determined to reduce the world to order, the military ideal was to achieve maximum reliability and efficiency by training troops to operate in a machinelike manner. This implied much tighter discipline and organization, which in turn required a shift toward the type of regular, professional forces that alone were capable of achieving them.

About 1670 the bayonet was invented, causing pikes to be discarded and homogeneous infantry to be created (though the expression “to trail a pike” lingered for another century). Apart from predicaments when it had to form squares in order to confront attacking cavalry, infantry now fought in very long, thin formations. Throughout the 18th century a lively debate was carried on concerning the best ways to employ these formations, but basically each side organized its forces in two lines separated by perhaps 300 to 400 yards and moving forward one behind the other. Though the precise arrangements varied from one army to the next, inside each line the units were organized by platoon, company, and battalion. Advancing toward each other, each side would hold its fire for as long as possible in order to close range and obtain a better aim, and then, acting upon the word of command, the opposing lines would fire salvo after salvo into each other. The final step consisted of fixing bayonets and storming the enemy—although, since one side usually broke, actual hand-to-hand fights seem to have been rare. Flank protection was provided by light cavalry such as dragoons or hussars, which were introduced in force between 1690 and 1740. Heavy cavalry would be held in reserve, ready to strike when a gap was created or a flank presented itself. During the second half of the 18th century another type of cavalry, the lancers, was added specifically to root out gunners hiding under their cannons’ barrels.

The first cannon were slow-firing devices much too cumbersome to take part in tactical maneuvers, and indeed so heavy were they that until about 1500 they were not even provided with wheels. Even then, the standard method was to position the guns in the intervals between units and in front of the advancing lines. This permitted them to open the battle but subsequently forced them to fall silent as the army advanced and left the gunners behind. To solve this problem, there was a steady tendency to make artillery smaller and more mobile, from the “leather guns” fielded by Gustav Adolf in the 1630s to the horse artillery developed after 1760—by which time anything heavier than 12-pounders (that is, firing 12-pound [5.4-kilogram] balls) was no longer considered suitable for battlefield use. It then became possible to move the guns during the combat, massing them against selected sections of the enemy front as the tactical situation might require. This flexibility, however, was offset by the fact that 18th-century linear formations were almost impossible to turn around. Hence, the really artistic touch consisted of so arranging things as to fall with one’s whole force upon one of the enemy’s flanks; witness the great victories that Frederick the Great, employing his so-called oblique order, achieved at Rossbach and Leuthen in 1757.