There has always been slavery throughout history, but the significant difference between slavery within the US historical context and the world historical context is this: The US based slavery system was based purely on skin color. Prior to importing slaves from Africa, the early settlers imported indentured servants from England who signed 7 year contracts and upon the completion of the contracts were promised some land that they can raise. The working conditions were not always favorable nor were the owners always the most benevolent nor were the contracts fulfilled at term. This lead a lot of the WHITE indentured servants to run away which presented a problem to the landlords, with the runaway servants being able to blend in with the crowd, the landlords sought a new solution. AFRICANS! The Africans were already in the slave trade for commercial purposes, and with the import of the Africans to work the land, even if they chose to run away, they were quickly recaptured due to their inability to mix in with the crowd unlike their predecessors (the indentured servants). The United States was also under Commonwealth Law (aka Common Law), due to the atrocities committed by the slave owners, Common Law no longer applied to slaves such as children of a citizen of the Commonwealth were not recognized. The Natives were viewed as occupiers of the land they wished to bum rush and even the Constitution stipulated "WHITE MEN" were allowed to be citizens...After the emancipation proclamation, the former slaves were considered 6/10 of a person and the Native Americans 1/8 of a person thus making any sort of census near impossible and by not being perceived as a whole person this allowed for the continued denigration and maltreatment of these victims and why to this day in the United States the residue of this perception that they are not considered as persons still resonate today.
In historical slavery, whether it was Greece, Africa, Asia or any other parts of the world, slaves were mostly a spoil that went to the victor, and if you were a slave then your children were slaves, in Africa some slaves were held in rather high regards. A great example of this was Joseph, who was sold off by his brothers as a slave to the Egyptians, who later became one of the king's most trusted adviser. Slavery, in this sense was never based exclusively on race, creed, religion, etc. but purely commercial and a spoils of war basis.
So until the recognition that the whites of the United States still acknowledge there is still some subconscious perception of non-whites as less than a full human being, race relations in the United States will stay at the standstill it is to this day. As my theory of "Racial Categoricalism" states, every ethnicity and race is categorized and are compartmentalized and if we don't fit the picture frame we are not accepted.