Army
Structure: Armies are always divided into various specialties, according to the mission, role, and training of individual units, and sometimes individual soldiers within a unit. 
Some of the groupings common to all armies include the following:

Infantry
Armoured corps
Artillery corps
Signal corps
Special forces
Military police
Medical corps

History: 

India: The Battle of the Ten Kings, a Hindu Aryan king named Sudas defeated an alliance of ten kings and their supportive chieftains. During the Iron Age, the Maurya and Nanda Empires had one of the largest armies in the world, the peak being approximately over 600,000 Infantry, 30,000 Cavalry, 8,000 War-Chariots and 9,000 War Elephants not including tributary state allies. In the Gupta age, large armies of longbowmen were recruited to fight off invading horse archer armies. Elephants, pikemen and cavalry were other featured troops.

China: The states of China raised armies for at least 1000 years before the Spring and Autumn Annals. By the Warring States period, the crossbow had been perfected enough to become a military secret, with bronze bolts that could pierce any armor. Thus any political power of a state rested on the armies and their organization. China underwent political consolidation of the states of Han (韓), Wei (魏), Chu (楚), Yan (燕), Zhao (趙) and Qi (齊), until by 221 BCE, Qin Shi Huang (秦始皇帝), the first emperor of the Qin dynasty, attained absolute power. This first emperor of China could command the creation of a Terracotta Army to guard his tomb in the city of Xi'an (西安), as well as a realignment of the Great Wall of China to strengthen his empire against insurrection, invasion and incursion.
Sun Tzu's The Art of War remains one of China's Seven Military Classics, even though it is two thousand years old. Since no political figure could exist without an army, measures were taken to ensure only the most capable leaders could control the armies. Civil bureaucracies (士大夫) arose to control the productive power of the states, and their military power.

Sparta: The Spartan Army was one of the earliest known professional armies. Boys were sent to a barracks at the age of seven or eight to train for becoming a soldier. At the age of thirty, they were released from the barracks and allowed to marry and have a family. After that, men devoted their lives to war until their retirement at the age of 60. The Spartan Army was largely composed of hoplites, equipped with arms and armor nearly identical to each other. Each hoplite bore the Spartan emblem and a scarlet uniform. The main pieces of this armor were a round shield, a spear and a helmet.

Ancient Rome: The Roman Army had its origins in the citizen army of the Republic, which was staffed by citizens serving mandatory duty for Rome. Conscription remained the main method through which Rome mustered forces until the end of the Republic. The army eventually became a professional organization largely of citizens, who would served continuously for 25 years before being discharged.
The Romans were also noted for making use of auxiliary troops, non-Romans who served with the legions and filled roles that the traditional Roman military could not fill effectively, such as light skirmish troops and heavy cavalry. After their service in the army they were made citizens of Rome and then their children were citizens also. They were also given land and money to settle in Rome. In the Late Roman Empire, these auxiliary troops, along with foreign mercenaries, became the core of the Roman Army; moreover, by the time of the Late Roman Empire tribes such as the Visigoths were paid to serve as mercenaries.

Medieval Europe: In the earliest Middle Ages it was the obligation of every aristocrat to respond to the call to battle with his own equipment, archers, and infantry. This decentralized system was necessary due to the social order of the time, but could lead to motley forces with variable training, equipment and abilities. The more resources the noble had access to, the better his troops would be.
Initially, the words "knight" and "noble" were used interchangeably as there was not generally a distinction between them. While the nobility did fight upon horseback, they were also supported by lower class citizens – and mercenaries and criminals – whose only purpose was participating in warfare because, most often than not, they held brief employment during their lord's engagement. As the Middle Ages progressed and feudalism developed in a legitimate social and economic system, knights started to develop into their own class with a minor caveat: they were still in debt to their lord. No longer primarily driven by economic need, the newly established vassal class were, instead, driven by fealty and chivalry.
As central governments grew in power, a return to the citizen armies of the classical period also began, as central levies of the peasantry began to be the central recruiting tool. England was one of the most centralized states in the Middle Ages, and the armies that fought in the Hundred Years' War were, predominantly, composed of paid professionals.
In theory, every Englishman had an obligation to serve for forty days. Forty days was not long enough for a campaign, especially one on the continent.
Thus the scutage was introduced, whereby most Englishmen paid to escape their service and this money was used to create a permanent army. However, almost all high medieval armies in Europe were composed of a great deal of paid core troops, and there was a large mercenary market in Europe from at least the early 12th century.
As the Middle Ages progressed in Italy, Italian cities began to rely mostly on mercenaries to do their fighting rather than the militias that had dominated the early and high medieval period in this region. These would be groups of career soldiers who would be paid a set rate. Mercenaries tended to be effective soldiers, especially in combination with standing forces, but in Italy they came to dominate the armies of the city states. This made them considerably less reliable than a standing army. Mercenary-on-mercenary warfare in Italy also led to relatively bloodless campaigns which relied as much on maneuver as on battles.
In 1439 the French legislature, known as the  Estates General (French: états généraux), passed laws that restricted military recruitment and training to the king alone. There was a new tax to be raised known as the taille that was to provide funding for a new Royal army. The mercenary companies were given a choice of either joining the Royal army as compagnies d'ordonnance on a permanent basis, or being hunted down and destroyed if they refused. France gained a total standing army of around 6,000 men, which was sent out to gradually eliminate the remaining mercenaries who insisted on operating on their own. The new standing army had a more disciplined and professional approach to warfare than its predecessors. The reforms of the 1440s,  eventually led to the French victory at Castillon in 1453, and the conclusion of the Hundred Years' War. By 1450 the companies were divided into the field army, known as the grande ordonnance and the garrison force known as the petite ordonnance.

Early modern: First nation states lacked the funds needed to maintain standing forces, so they tended to hire mercenaries to serve in their armies during wartime. Such mercenaries typically formed at the ends of periods of conflict, when men-at-arms were no longer needed by their respective governments.
The veteran soldiers thus looked for other forms of employment, often becoming mercenaries. Free Companies would often specialize in forms of combat that required longer periods of training that was not available in the form of a mobilized militia.
As late as the 1650s, most troops were mercenaries. However, after the 17th century, most states invested in better disciplined and more politically reliable permanent 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 centralization of government functions caused a series of revolts across Europe such as the Fronde in France and the English Civil War.
In many countries, the resolution of this conflict was the rise of absolute monarchy. Only in England and the Netherlands did representative government evolve as an alternative. From the late 17th century, states learned how to finance wars through long term low interest loans from national banking institutions. The first state to master this process was the Dutch Republic. This transformation in the armies of Europe had great social impact. The defense of the state now rested on the commoners, not on the aristocrats. However, aristocrats continued to monopolize 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.