Slavery was an important part of the ancient world, and it was an integral piece of Roman daily life and the economy. Though slavery was practiced all over the Mediterranean, and was abundant in the east, its impact in other places was not felt nearly as much as it was in Rome and her Empire. As the Romans consolidated their position on the Italian peninsula and began the systematic conquest of the Mediterranean region, millions of slaves were incorporated into Rome and the Italian countryside.
When the Romans were farmers and shepherds, slaves were used for farm work. Citizens were often away at war and slaves were necessary to keep the farms going. So the use of slaves gradually increased, until they were more numerous than free men who worked for pay. Eventually competition with slave labour determined wages and living conditions of free workmen.
Slavery was, of course, very destructive morally. It resulted in the love of luxury and indolence which later characterized a Roman. The Romans lost their old virtues of simplicity, frugality and temperance. And as they lost their strength of character in a life of ease and luxury, their sway over the civilized world decreased.
We cannot be certain of how many slaves there were in Rome at any given time. But we can interpret generalities; for example, that slaves were few in early times is shown by the fact that they were distinguished by name. However, in later times, we know that certain generals were said to dispose of huge numbers of slaves. Horace implies that even a gentleman in moderate circumstances had to have at least ten slaves (the number he possessed). We hear of some individuals who had unbelievable numbers of slaves. Sometimes slaves in a certain household were divided into groups of ten, so we know that there were hordes of these, at least during the Empire.
Most slaves were war captives. They were sold soon after they were taken in order to avoid the