An organized force of people armed for fighting on land. Armies came into existence with the earliest states and underpinned the great empires of antiquity: Egypt, Babylon, and Assyria. The essential components of armies in early history were infantry, with some chariots, and cavalry. In ancient Greece the tendency towards greater professionalism reached its climax with the Macedonian army of Alexander the Great. From this time on, the development of siege techniques was an important part of military practice. The generals of Carthage, especially Hannibal, hired mercenaries to great effect in their forces, but it was the armies of Rome, gradually evolving into fully professional standing forces, which dominated Europe from the 2nd century bc to the 5th century ad. Less organized but swiftly moving armies then came to the fore in the Dark Ages, from those of Attila the Hun to the Mongols. In Europe in the Middle Ages the limitations of the heavily armoured mounted knight were finally exposed by Swiss infantry armed with pikes or halberds and English infantry armed with longbows. The use of mercenaries (see condottiere) again became commonplace.

The major advances of the 15th and 16th centuries were the invention of gunpowder and the development of cannon. Organization, discipline, and further advances in weaponry led to the creation of highly efficient armies, most notably those of Frederick II (the Great) of Prussia. In the late 18th century, European armies were mainly of mercenaries recruited (often under pressure) and trained by a professional officer class. The first conscript armies were recruited in France to fight the Revolutionary and Napoleonic Wars. During the 19th century most European countries adopted a system of conscription of young men to train and serve for about two years. (Britain only enforced conscription in 1916–18 and again between 1939 and 1959.) European armies played an essential role in 19th‐ and early 20th‐century imperialism, their superior fire‐power enabling them to dominate the peoples of Africa and Asia. The American Civil War (1861–65) saw large armies of the Union (the North) and the Confederacy (the South) engaged in a struggle in which railways were crucial for movement of troops, and new infantry weapons, such as the breech‐loading rifle and the repeating carbine, were developed. By the time of the Franco‐Prussian War in 1870–71 heavy artillery was developing, but infantry and cavalry tactics remained little changed until World War I, when motor transport and heavier artillery developed. Even then, armies were slow to adapt to armoured vehicles and the massed infantry attacks of its battles still used rifle, bayonet, and hand‐grenade as their basic weapons, now pitched against machine guns. By World War II armies were fully motorized and tanks played a major part in the North African Campaign and at the Eastern Front. This mobility required large back‐up fuel and maintenance services. Basic infantry tactics still remained essential (even though the rifle was being replaced by the semiautomatic or automatic submachine gun), especially in the jungle warfare of the Burma Campaign. They remained so for later campaigns in Korea, Vietnam, and the Falklands. In the Cold War balance of power, large armies of NATO and the Warsaw Pact continued to face one another in Europe, armed with both conventional weapons and missiles. Allied victory in the Gulf War was achieved through massive tank deployment. Since the end of the Cold War the armies of UN member nations have increasingly been combined to form multinational peacekeeping and ‘rapid reaction’ forces.