Slaves turned into stateless people

The world put an end to slavery in the 18^th and 19^th centuries because by then machines driven by fossil fuels could do a better job than slave labor. 

In the ancient world the chief source of slave labor was populations of defeated peoples that were taken into captivity and were integrated into a foreign country. Slaves in the ancient world turned into what in the modern world became stateless people. 

With the abolition of slavery, the world had to find some other means of dealing with stateless peoples. Concentration camps, genocides, and extermination camps seem, for the time being, to have been the result.