Generations of warfare

The concept of four "generations" in the history of modern warfare was created by a team of United States analysts, including William S. Lind, for the purpose of an argument for "the changing face of war" entering into a "fourth generation".
First generation.
In 1648, at the end of the Thirty Years' War, the Treaty of Westphalia gave a practical sovereignty to the German states, which until then were semi-independent components of the Holy Roman Empire. This more firmly established the sovereignty of the nation-state, which meant, among other things, that governments would have exclusive rights to organize and maintain their own militaries. Before this time, many armies and nations were controlled by religious orders and many wars were fought in mêlée combat, or subversively through bribery and assassination. The first generation of modern warfare was intended to create a straightforward and orderly means of waging war. 
Alternatively, it has been argued that the Peace of Westphalia did not solidify the power of the nation-state, but that the Thirty Years' War itself ushered in an era of large-scale combat that was simply too costly for smaller mercenary groups to carry out on their own. According to this theory, smaller groups "chose" to leave mass combat—and the expenses associated with it—in the domain of the nation-state. 
The increased accuracy and speed of the rifled musket and the breech-loader marks the end of first generation warfare; the concept of vast lines of soldiers meeting face to face became impractical due to the heavy casualties that could be sustained. Because these technologies were adopted gradually throughout the Americas and Europe, the exact end of the first generation of modern warfare depends on the region, but all world powers had moved on by the latter half of the 19th century.
In order to create a more controlled environment for warfare a military culture was developed that, in many ways, is still visible in the armed forces of today. Specially crafted uniforms set soldiers apart from the general populace. 
An elaborate structure of rank was developed to better organize men into units. Rules for military drill were perfected, allowing line and column maneuvers to be executed with more precision, and to increase the rate of fire in battle.
Control of media information release during the war and production of counterfeit money in order to devaluate enemy's economy were used for the first time during Napoleonic wars.
Examples:
Second generation.
In the 19th century, the invention of the breech-loading rifled musket meant longer range, greater accuracy, and faster rate of fire. Marching ranks of men straight into a barrage of fire from such weapons would cause tremendous rates of casualties, so a new strategy was developed. 
Second generation warfare still maintained lines of battle but focused more on the use of technology to allow smaller units of men to maneuver separately. These smaller units allowed for faster advances, less concentrated casualties, and the ability to use cover and concealment to advantage.
To some degree, these concepts have remained in use even as the next generations have arisen, so the end of the second generation is not as clearly defined as that of the first. The development of the blitzkrieg highlighted some of the flaws of static firing positions and slow-moving infantry, so this can be considered the beginning of the end for the second generation, at least as the dominant force in military strategy.
The contributions of the second generation were responses to technological development. The second generation saw the rise of trench warfare, artillery support, more advanced reconnaissance techniques, extensive use of camouflage uniforms, radio communications, and fireteam maneuvers.
Examples:
Third generation.
The use of blitzkrieg during the German invasion of France first demonstrated the power of speed and maneuverability over static artillery positions and trench defenses. Through the use of tanks, mechanized infantry, and close air support, the Germans were able to quickly break through linear defenses and capture the rear.
The emphasis on maneuvering and speed to bypass enemy engagement remains a common strategy throughout the world, and collapsing an enemy's defenses by striking at deeper targets is—in a somewhat different way—a major strategy in fourth generation warfare.
The contributions of the third generation were based on the concept of overcoming technological disadvantage through the use of clever strategy. As linear fighting came to an end, new ways of moving faster began to appear.
The emphasis on mobility moved from heavy armor to greater speed, the development of the helicopter allowed insertions in hostile territory, and advanced missile technology allowed forces to bypass enemy defenses and strike at targets from great distances.
The speed inherent in these methods necessitated a greater degree of independence allowed to the units on the front lines.
Greater trust needed to be placed in junior officers commanding sub-units by higher-ranking officers—a belief that they could adequately achieve their objectives without micromanagement from higher ranking commanders in command headquarters.
Smaller units were allowed greater decision flexibility to deal with changing situations on the ground, rather than have decisions made for them by commanders who were distant from the front. This began to break down the regimented culture of order that was so important in previous theoretical eras of military command and control.
Examples:
Fourth generation.
Fourth-generation warfare is characterized by a blurring of the lines between war and politics, combatants and civilians. The term was first used in 1989 by a team of United States analysts, including William S. Lind, to describe warfare's return to a decentralized form. In terms of generational modern warfare, the fourth generation signifies the nation states' loss of their near-monopoly on combat forces, returning to modes of conflict common in pre-modern times.
The simplest definition includes any war in which one of the major participants is not a state but rather a violent non-state actor. Classical examples, such as the slave uprising under Spartacus or the mercenary uprising that occurred in Carthage after the first Punic War, predate the modern concept of warfare and are examples of this type of conflict.
Fourth generation warfare is defined as conflicts which involve the following elements:
Fourth-generation warfare theory has been criticized on the grounds that it is "nothing more than repackaging of the traditional clash between the non-state insurgent and the soldiers of a nation-state."