See also: Slavery in ancient Greece
Records of slavery in Ancient Greece go as far back as Mycenaean Greece. The origins are not known, but it appears that slavery became an important part of the economy and society only after the establishment of cities. Slavery was common practice and an integral component of ancient Greece, as it was in other societies of the time, including ancient Israel and early Christian societies. It is estimated that in Athens, the majority of citizens owned at least one slave. Most ancient writers considered slavery not only natural but necessary, but some isolated debate began to appear, notably in Socratic dialogues. The Stoics produced the first condemnation of slavery recorded in history.
During the 8th and the 7th centuries BC, in the course of the two Messenian Wars, the Spartans reduced an entire population to a pseudo-slavery called helotry. According to Herodotus (IX, 28–29), helots were seven times as numerous as Spartans. Following several helot revolts around the year 600 BC, the Spartans restructured their city-state along authoritarian lines, for the leaders decided that only by turning their society into an armed camp could they hope to maintain control over the numerically dominant helot population. In some Ancient Greek city states about 30% of the population consisted of slaves, but paid and slave labor seem to have been equally important.
See also: Slavery in ancient Rome
Romans inherited the institution of slavery from the Greeks and the Phoenicians. As the Roman Republic expanded outward, it enslaved entire populations, thus ensuring an ample supply of laborers to work in Rome's farms and households. The people subjected to Roman slavery came from all over Europe and the Mediterranean. Such oppression by an elite minority eventually led to slave revolts; the Third Servile War led by Spartacus was the most famous and severe. Greeks, Berbers, Germans, Britons, Slavs, Thracians, Gauls (or Celts), Jews, Arabs and many more ethnic groups were enslaved to be used for labor, and also for amusement (e.g. gladiators and sex slaves). If a slave ran away, he was liable to be crucified. By the late Republican era, slavery had become a vital economic pillar in the wealth of Rome. In the Roman Empire, probably over 25% of the empire's population, and 30 to 40% of the population of Italy was enslaved.
Celtic tribes of Europe are recorded by various Roman sources as owning slaves. The extent of slavery in prehistorical Europe is not well known, however.
Main article: Slavery in medieval Europe
The chaos of invasion and frequent warfare also resulted in victorious parties taking slaves throughout Europe in the early Middle Ages. St. Patrick, himself captured and sold as a slave, protested against an attack that enslaved newly baptized Christians in his "Letter to the Soldiers of Coroticus". As a commonly-traded commodity, like cattle, slaves could become a form of internal or trans-border currency. Slavery during the Early Middle Ages had several distinct sources.
The Vikings raided across Europe, but took the most slaves in raids on the British Isles and in Eastern Europe. While the Vikings kept some slaves as servants, known as thralls, they sold most captives in the Byzantine or Islamic markets. In the West their target populations were primarily English, Irish, and Scottish, while in the East they were mainly Slaves. The Viking slave-trade slowly ended in the 11th century, as the Vikings settled in the European territories they had once raided. They converted serfs to Christianity and themselves merged with the local populace.
Medieval Spain and Portugal saw almost constant warfare between Muslims and Christians. Al-Andalus sent periodic raiding expeditions to loot the Iberian Christian kingdoms, bringing back booty and slaves. In a