An army is a that portion of a nation's military charged with land operations. An army can incorporate standing elements, which are composed of full-time soldiers, and augment its size and breadth with reserve elements that have been trained beforehand and that can be activated during time of war or national emergency.

Organized field armies, combining trained soldiers and technology date back to the Hittite civilization and pre-Ptolemaic Egyptian civilization of the 15th century B.C., during the Iron age.http://www.armchairgeneral.com/shock-tactics-on-the-ancient-battlefield.htm

It was at this time that the first shock weapon was brought into the fray in the form of the chariot. The chariot would remain a key shock weapon, coming out of and back into favor into the classical period of history with the armies of Ancient Greece, Persia and Rome.http://www.armchairgeneral.com/shock-tactics-on-the-ancient-battlefield.htm

In the third century B.C., Greek armies began to develop equine units for their field armies. From this base, cavalry units were developed, which were brought and used to great efficiency in later centuries by the Roman Empire. Cavalry became the newest weapon for decisive power and speed.http://www.armchairgeneral.com/shock-tactics-on-the-ancient-battlefield.htm

Roman discipline in their legions brought about many victories, often against overwhelming numbers. Many military protocols now common with armies around the world trace back their roots to the old Roman legions.http://www.globalsecurity.org/military/intro/legion.htm

Specialized branches for the army, precursors to general staffs, the salute, standard bearers for field units, specialized field communications all trace their initial successful fruition to the Roman legions.http://www.globalsecurity.org/military/intro/legion.htm

During the Medieval period, the use of gun powder and artillery was introduced to armies, as well as pikes, and heavy armored cavalry (exemplified by the mounted knight). In spite of this infusion of technology, army command and organization development remained fairly stagnant in the West since the demise of the Roman Empire.http://www.nipissingu.ca/department/history/muhlberger/orb/OVC5S4.HTM

From the Seven Years War (1756-1763) onward, in which Prussia was able to withstand repeated military efforts against her from a coalition of countries which included France, Russia and Sweden, the Prussian king Frederick II spurred the rediscovery of precision drilling in military tactical movements. This spawned a movement through efficiency and speed of maneuver on the field, which would later come to full fruition with Napoleon Bonaparte four decades later.

Napoleon built his 'Grand Armee' around the cor ideas of Frederick II of speed, size and maneuver. Napoleon went further and developed a command and staff structure suitable for this kind of army. With strategy, tactics and command leadership all coalescing in one given point, huge armies of over 500,000 could now take to the field.http://www.napoleon-series.org/military/organization/c_rma.html

Out of this also came the organization of a hierarchical structure in army units, that have been in use to this day, with slight variations along the way.

• From highest to Lowest http://en.wikipedia.org/wiki/Military_organization#Commands.2C_formations.2C_and_units
  

• Army Group (250,000+ men)

• Army (60,000–100,000+ men)

• Corps (30,000–80,000 men)

• Division (10,000–20,000 men)

• Brigade (2000–5000 men)

• Battalion (300–1000 men)

• Company (70–250 men)

• Platoon (25–60 men)

• Squad (8–16 men)

• Team (2–5 men)

China:

Total Land-Based Weapons: 31,300
Tanks: 8,200
Armored Personnel Carriers: 5,000
Towed Artillery: 14,000
Self-Propelled Guns: 1,700
Multiple Rocket Launch Systems: 2,400
Mortars: 16,000
Anti-Tank Guided Weapons: 6,500
Anti-Aircraft Weapons: 7,700

United States:

Total Land-Based Weapons: 29,920
Towed Artillery: 5,178

India:

Total Land-Based Weapons: 10,340
Tanks: 3,898
Armored Personnel Carriers: 317
Towed Artillery: 4,175
Self-Propelled Guns: 200
Multiple Rocket Launch Systems: 150
Anti-Aircraft Weapons: 2,424

Russia:

Total Land-Based Weapons: 79,985
Tanks: 22,800
Armored Personnel Carriers: 9,900
Towed Artillery: 13,585
Self-Propelled Guns: 6,010
Multiple Rocket Launch Systems: 4,350
Mortars: 6,100

North Koreahttp://www.globalfirepower.com/country-military-strength-detail.asp?country_id=North-Korea:

Total Land-Based Weapons: 16,400
Tanks: 3,500
Armored Personnel Carriers: 2,500
Towed Artillery: 3,500
Self-Propelled Guns: 4,400
Multiple Rocket Launch Systems: 2,500
Mortars: 7,500
Anti-Aircraft Weapons: 11,000