Early modern warfare
However, the main reason was that armies were now much bigger, but logistical support for them was inadequate. This meant that armies tended to devastate civilian areas in an effort to feed themselves, causing famines and population displacement. This was exacerbated by the increasing length of conflicts, such as the Thirty Years' War and Eighty Years' War, when this period saw the size and scale of warfare greatly increase. The number of combatants involved escalated steadily from the mid 16th century and they fought over areas subjected to repeated devastation. For this reason, the wars of this era were among the most lethal before the modern period.
For example, the Thirty Years' War and the contemporary Wars of the Three Kingdoms, were the bloodiest conflicts in the history of Germany and Britain respectively before World War I. Another factor adding to bloodshed in war was the lack of a clear set of rules concerning the treatment of prisoners and non-combatants. While prisoners were usually ransomed for money or other prisoners, they were sometimes slaughtered out of hand - as at the Battle of Dungans Hill in 1647.
One of the reasons for warfare's increased impact was its indecisiveness. Armies were slow moving in an era before good roads and canals. Battles were relatively rare as armies could manoeuvre for months, with no direct conflict. In addition, battles were often made irrelevant by the proliferation of advanced, bastioned fortifications. To control an area, armies had to take fortified towns, regardless of whether they defeated their enemies' field armies. As a result, by far the most common battles of the era were sieges, hugely time-consuming and expensive affairs. Storming a fortified city could result in massive casualties and cities which did not surrender before an assault were usually brutally sacked -for example Magdeburg in 1631 or Drogheda in 1649. In addition, both garrisons and besiegers often suffered heavily from disease.

The indecisive nature of conflict meant wars were long and endemic. Conflicts stretched on for decades and many states spent more years at war than they did at peace. The Spanish attempt to reconquer the Netherlands after the Dutch Revolt became bogged down in endless siege warfare. The expense caused the Spanish monarchy to declare bankruptcy several times, beginning in 1577.
The changes in warfare eventually made the mercenary forces of the Renaissance and Middle Ages obsolete. However this was a gradual change. As late as the Thirty Years' War (1618–1648), most troops were mercenaries. However, after this conflict, most states invested in better disciplined and more ideologically inspired troops. For a time mercenaries became important as trainers and administrators, but soon these tasks were also taken by the state. The massive size of these armies required a large supporting force of administrators. The newly centralized states were forced to set up vast organized bureaucracies to manage these armies, which some historians argue is the basis of the modern bureaucratic state.
The combination of increased taxes and increased centralisation of government functions caused a series of revolts across Europe such as the English Civil War and the Fronde in France. In many countries, the resolution of this conflict was the rise of monarchical absolutism. Only in England and the Netherlands did representative government evolve as an alternative. From the late 17th century, states started financing wars through long term low interest loans from national banking institutions like the Bank of England. The first state to take full advantage of this process was the Dutch Republic.

This transformation in the armies of Europe had great social impact. J. F. C. Fuller famously stated that "the musket made the infantryman and the infantryman made the democrat". This argument states that the defence of the state now rested on the common man, not on the aristocrats. Revolts by the underclass, that had routinely been defeated in the Middle Ages, could now conceivably threaten the power of the state. However, aristocrats continued to monopolise the officer corps of almost all early modern armies, including their high command.
Moreover, popular revolts almost always failed unless they had the support and patronage of the noble or gentry classes. The new armies, because of their vast expense, were also dependent on taxation and the commercial classes who also began to demand a greater role in society. The great commercial powers of the Dutch and English matched much larger states in military might.
As any man could be quickly trained in the use of a musket, it became far easier to form massive armies. The inaccuracy of the weapons necessitated large groups of massed soldiers. This led to a rapid swelling of the size of armies.
For the first time huge masses of the population could enter combat, rather than just the highly skilled professionals. It has been argued that the drawing of men from across the nation into an organized corps helped breed national unity and patriotism, and during this period the modern notion of the nation state was born. However, this would only become apparent after the French Revolutionary Wars. At this time, the levée en masse and conscription would become the defining paradigm of modern warfare.
Before then, however, most national armies were in fact composed of many nationalities. For example, although the Swedish Army under Gustavus Adolphus was originally recruited by a kind of national conscription, the losses of the Thirty Years' War meant that by 1648 over 80% of its troops were foreign mercenaries. In Spain, armies were recruited from all the Spanish European territories including Spain, Italy, Wallonia and Germany. The French recruited soldiers from Germany, Switzerland and elsewhere as well as from France. Britain recruited Hessian troops until the late 18th century. Irish Catholics made careers for themselves in the armies of many European states (See the Flight of the Wild Geese).

Infantry: Column - This formation was typically used while marching, although with sufficient will and mass it was effective at breaking through line formations, albeit with heavy casualties.
Line - A simple two- or three-rank deep line formation allowed most muskets to be brought to bear and was the most commonly used battle formation. Often the first rank would kneel after firing to allow the second rank to fire.
Skirmishers - Skirmishers were not a common infantry unit until late in the 18th century. Light infantry would advance and be the first to fire to draw the enemy to attack, while also probing the flanks. In later eras, sharpshooters would not only target common soldiers, but also officers so that the men were without leadership.
Square - This formation was used against cavalry. Bayonets would be fixed, the first line would kneel with their muskets angled upward (much like a pike.) The second and third lines would fire at the cavalry when it came close. This formation was very ineffective when faced with combined cavalry and infantry, or artillery fire in the case of plain squares.

Cavalry: The rise of gunpowder reduced the importance of the once dominant heavy cavalry, but it remained effective in a new role into the 19th century. The cavalry, along with the infantry, became more professional in this period but it retained its greater social and military prestige than the infantry. Light cavalry was introduced for skirmishing and scouting because of its advantage in speed and mobility. The new types of cavalry units introduced in this period were the dragoons or mounted infantry.
Dragoons were intended to travel on horseback but fight on foot and were armed with carbines and pistols. Even orthodox cavalry carried firearms, especially the pistol, which they used in a tactic known as the caracole. Cavalry charges using swords on undisciplined infantry could still be quite decisive, but a frontal charge against well-ordered musketeers and pikemen was all but futile. Cavalry units, from the 16th century on, were more likely to charge other cavalry on the flanks of an infantry formation and try to work their way behind the enemy infantry. When they achieved this and pursued a fleeing enemy, heavy cavalry could still destroy an enemy army. Only specialised cavalry units like winged hussars armed with long lances could break pikemen lines, but this was rather an exception. After wars with Polish-Lithuanian Commonwealth, when he fought often against superior mounted troops, King Gustavus II Adolphus started using successfully cavalry melee charge more often instead of caracole like during Battle of Breitenfeld. The cavalry charge remained an important part of battle tactics for the rest of 17th century and until the modern area, and its shock value could be decisive when implemented properly like during Battle of Vienna (1683).
However, the power formerly wielded by a heavy cavalry-focused army was at an end. For the first time in millennia, the settled people of the agricultural regions could defeat the horse peoples of the steppe in open combat. The power of the Mongols was broken in Russia and, no longer threatened from the east, Russia began to assert itself as a major force in European affairs. Never again would nomads from the east threaten to overrun Europe or the Middle East. In the Siege of Kazan (1552), Russia had employed artillery, sappers, cavalry and infantry armed with arquebus (Streltsy), while the Khanate of Kazan had only employed cavalry. The use of sappers proved decisive.
The one exception to this was the Ottoman Empire, which had been founded by Turkish horsemen. The Ottomans were some of the first to embrace gunpowder artillery and firearms and integrated them into their already formidable fighting abilities.