Military recruitment is the means by which a person or nation raises an army for the purpose of war. Such armies can be raised in order to defend the person or nation or for the army to attack others. There are several methods used for military recruitment including by oath, conscription, through volunteering and hiring mercenaries. Not all nations have a standing army and several, including Costa Rica since 1949 and Liechtenstein since 1868, have no army whatsoever.
The old Anglo-Saxon and Early English armies in the Early Middle Ages used a system of oaths to raise an army. In times of war, such as when Harold II of England marched north to face the Vikings at Stanford Bridge and then south to face the Normans at Hastings in 1066, the King would call in oaths from local communities. Such oaths were balanced so that enough able-bodied men remained behind in order to keep farming the land.
Feudalism, as introduced by the Normans to England, and common in Europe during the Middle Ages, built on the oath system. Rules were established by monarchs as to how they and the nobility could begin military recruitment. The King of England, for example, would have to call a parliament to get permission to raise a tax. He would then use the tax to buy an army. Most peasant soldiers had little or no choice in the matter.
Conscription is another form of forced military recruitment. The nation draws a list of able-bodied men of the right age and forces them to enlist in the army. There were a number of punishments for deserting duty and by evading conscription. France used conscription during the Napoleonic Wars, but still lost to a voluntary British army a tenth of its size. America first used a national conscription system in the civil war and finally discontinued it in 1973; however, it can be reactivated at any time if needed.
Mercenaries are an altogether different kind of military recruitment. The other kinds of recruitment take normal citizens or subjects and train them in warfare. When a person or nation uses mercenaries, they are buying a ready-made army that is fully equipped and trained for the job. These are euphemistically called Private Military Companies (PMCs) in America.
There are several examples of mercenaries being used in world history. The Romans employed barbarian tribes as auxiliaries or foederati during the Roman Empire. Xenophon was a mercenary when his army of Greek hoplites was paid to help depose a Persian Emperor by the Emperor’s brother. In 2011, Colonel Gaddafi of Libya employed foreign mercenaries to put down a civil rebellion.