Slavery is an extremely old concept, a concept meaning that a human being can be owned by another human being, just like any other item. People have held slaves since ancient times - I think there has been slaves since man settled down for the first time; and that is a long time ago. It probably all started with two cavemen-tribes having a fight. Then some caveman got knocked out by a club, and when he woke up he found himself in the wrong camp! Well, then his captors probably wondered what to do with their prisoner of war (I suppose on such occasions they sometimes ate their prisoners, but lets presume these cavemen were a little civilised). And then some smart caveman suddenly got the great idea that they could keep the other caveman to work for them instead of Ogor, who got his head bashed in during the fight! And so slavery was born.
Maybe later the cavemen started to knock out people in fights on purpose, because they liked having people to work for them (yes, they were lazy in the stone-age too!). And then they started going to other tribes on the purpose of knocking out people to use as slaves. And then maybe they started trading these slaves for other nice things (you know, flint axes and such). And so slave trade was invented.
Slave trade just grew and grew during history. Almost all cultures had slaves. The Vikings had their "treller", the Aztec kept those prisoners of their numerous wars they didn’t sacrifice as slaves, the Romans had slaves for almost every purpose, and so on. During the middle ages the slaves weren’t exactly called slaves, but the poor peasants working at a great farm had no more freedom than most slaves. Anyway; usually slaves came from another culture, although sometimes you could find people of the same culture working as slaves to pay off a debt or make up for a crime. Among the Vikings people were often sentenced to "trell" for people they had committed a crime against. Still the majority of slaves were outsiders. I think that was because slaves were thought of as not-entirely-humans, and its easier to keep this view of those who are not of your own people.
Slave trade grew big after America was colonised . The need for cheap workers created a market for slaves. They were mostly brought from Africa, where they were easy to get. Slave trade was a barbaric practice - slaves were kept manacled in unsanitary conditions underneath the deck. Some slavers reckoned 0.5x0.5 m (or even less) of room for each slave. Needless to say the death rates under transport were enormous. But the slavers didn’t care. They made more than enough money on their filthy trade already.
In America the slaves were sold like cattle on auction. Some slaves might be lucky and get a kind master who treated his slaves in a humane way, but for most slaves life was endless toil and harsh treatment. And they were still slaves, no matter how nice their master was, and slaves weren’t real human beings in the eyes of most people. In Norway we often seem to think we had no part in the slave trade(just like we like to think we have no part in all the evils committed by humanity through history). But that’s not correct. There were several Danish-Norwegian slave ships, and they were no better than the others.
Slavery was eventually abolished, at least in the word. But it came at a high cost (not to say it wasn’t worth it). Slavery was one of the major causes of the American civil war. And although slavery was abolished in the US in 1865 it took along time before former slaves and their descendants were accepted as equals by American society. Among some they aren’t accepted yet.
And who said slavery is non-existent today? We know there are certain places in the world of today where slaves are kept, and not all of these are kept in Arabic or Asian countries.... And when we know there are slaves kept in countries that call themselves democratic, how can we not do anything about it and still be proud of our democracy?