Early modern warfare
Europe: 

Fortifications: The period from 1501 to 1800 saw a rapid advance in techniques of fortification in Europe. Whereas medieval castles had relied on high walls to keep out attackers, early modern fortifications had to withstand artillery bombardments. To do this, engineers developed a style of fortress known as the "Italian style" or trace Italienne. These had low, thick, sloping walls, that would either absorb or deflect cannon fire.
In addition, they were shaped like stars, with bastions protruding at sharp angles. This was to ensure that every bastion could be supported with fire from an adjacent bastion, leaving no dead ground for an attacker to take cover in. These new fortifications quickly negated the advantages cannon had offered to besiegers.
In response to the vulnerabilities of star forts, military engineers evolved a much simpler but more robust style of fortification. A polygonal fort is a fortification in the style that evolved around the middle of the 18th century, in response to the development of explosive shells. The complex and sophisticated designs of star forts that preceded them were highly effective against cannon assault, but proved much less effective against the more accurate fire of rifled guns and the destructive power of explosive shells. The polygonal style of fortification is also described as a "flankless fort". Many such forts were built in the United Kingdom and British Empire during the government of Lord Palmerston, and so they are also often referred to as Palmerston forts. Fort Tas-Silġ is an example of a British polygonal fort.

Firearms: The power of aristocracies vis à vis states diminished throughout Western Europe during this period. Aristocrats' 200- to 400-year-old ancestral castles no longer provided useful defences against artillery. The nobility's importance in warfare also eroded as medieval heavy cavalry lost its central role in battle. The heavy cavalry - made up of armoured knights - had begun to fade in importance in the Late Middle Ages. The English longbow and the Swiss pike had both proven their ability to devastate larger armed forces of mounted knights. However, the proper use of the longbow required the user to be extremely strong, making it impossible to amass very large forces of archers.
The proper use of the pike required complex operations in formation and a great deal of fortitude and cohesion by the pikemen, again making amassing large forces difficult. Starting in the early 14th century, armourers added plate-armour pieces to the traditional protective linked mail armour of knights and men-at-arms to guard against the arrows of the longbow and crossbow. By 1415, some infantrymen began deploying the first "hand cannons", and the earliest small-bore arquebuses, with burning "match locks", appeared on the battlefield in the later 15th century.

Decline of plate armour: In virtually all major European battles during a period of 250 years (1400 to 1650), many soldiers wore extensive plate armour; this includes infantrymen (usually pikemen) and almost all mounted troops. Plate armour was expected to deflect edged weapons and to stop an arquebus or pistol ball fired from a distance, and it usually did. The use of plate armour as a remedy to firearms tended to work as long as the velocity and weight of the ball remained quite low, but over time the increasing power and effectiveness of firearms overtook the development of defenses to counteract them, such that flintlock muskets (entering use after 1650) could kill an armoured man at a distance of even 100 yards (though with limited accuracy), and the armour necessary to protect against this threat would have been too heavy and unwieldy to be practical.
The flintlock musket, carried by most infantrymen other than pikemen after 1650, fired a heavier charge and ball than the matchlock arquebus. A recruit could be trained to use a musket in a matter of weeks. Operating a musket did not require the great physical strength of a pikeman or a longbowman or the fairly rare skills of a horseman. Unlike their arquebus predecessors, flintlock muskets could neutralize even the most heavily armoured cavalry forces.
Since a firearm requires little training to operate, a peasant with a gun could now undermine the order and respect maintained by mounted cavalry in Europe and their Eastern equivalents. Although well-smithed plate-armour could still prevent the penetration of gunpowder-weapons, by 1690 it had become no match for massed firearms in a frontal attack and its use ended, even among the cavalry. By the end of the 17th century, soldiers in the infantry and most cavalry units alike preferred the higher mobility of being completely unarmoured to the slight protection, but greatly lessened mobility, offered by donning the heavy plate armour of the period.

Transition to flintlock muskets: The arquebus, in use from 1410, was one of the first handheld firearms that were relatively light (it still required a stand to balance on) and a single person could operate one. One of these weapons was first recorded as being used in the Battle of Agincourt in 1415, although this was still very much a medieval battle. The term musket originally applied to a heavier form of the arquebus, which fired a shot that could pierce plate armour, though only at close range. In the 16th century it still had to be mounted on a support stick to keep it steady. The caliver was the lighter form of the arquebus. By 1600, armies phased out these firearms in favour of a new lighter matchlock musket. Throughout the 16th century and up until 1690, muskets used the matchlock design.
However, the matchlock design was superseded in the 1690s by the flintlock musket, which was less prone to misfires and had a faster reloading rate. By this time, only light-cavalry scouting units, "the eyes of the army", continued to wear front and back armour plates to protect themselves from distant or undisciplined musket-equipped troops.
While soldiers armed with firearms could inflict great damage on cavalry at a moderate distance, at close quarters the cavalry could slaughter the musket-armed infantry if they could break their formation and close to engage in melee combat. For many years infantry formations included a mix of troops armed with both firearms to provide striking power and pikes to allow for the defence of the arquebusiers or musketeers from a cavalry charge. The invention of the bayonet allowed the combining of these two weapons into one in the 1690s, which transformed the infantry into the most important branch of the early modern military—one that uniformly used flintlock muskets tipped with bayonets.

Nature of war: This period saw the size and scale of warfare greatly increase. The number of combatants involved escalated steadily from the mid 16th century and dramatically expanded after the 1660s. For example, Henry II of France, even in the dawn of religious unrest and inevitable violence, could amass an impressive 20,000 men in total for his 1550 decade of war against Habsburg Spain, but Louis XIV, Sun King with the highest population in the Kingdom of France and by extension Western Europe could deploy up to 500,000 men into the field by 1700 in the War of the Spanish Succession with more at stake. Moreover, wars and subsequent battles became increasingly deadly and pyrrhic in this period. The Battle of Fontenoy with advanced presence of Louis XV saw over 100,000 men deployed on both sides ending 20,000 lives, almost half of which were French, and despite a French victory, France herself did not keep Dutch territory gained as peace was desired for the bankrupt kingdom, a layman translation meaning almost 10,000 deaths were rendered obsolete by said king who witnessed the horrors from afar just 3 years later with very few battles left in the Austrian conflict. Cities that took months to siege could fall in mere days. European monarchs with bitter rivalries would invest many resources into intense warfare which often resulted in mass death and destruction of innocent populations, such as the Sack of Rome where the Supreme Pontiff's life was endangered, a symbolic attack against God and Christendom. The Italian Wars alone would threaten Europe's very existence. This may in part be attributed to improvements in weapons technology and in the techniques of using it (for example infantry volley fire).
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.