Slavery was used during extensively in WW2 by the Nazis for labour in the war industries.
Says in Wikipedia it was about 20% of the population of the Nazi occupied territories at its height.
WW2 Japan used slavery extensively.
China also has a forced labour system.
As indeed did the Soviets with their gulags.
So here we have another part of the jigsaw: forced labour in time of war…Hot or Cold War?
Being a PoW was often to be enslaved Eg. Japan, Nazi Germany (but in the case of Germany, PoWs were only enslaved if your country did not have German prisoners.)
Being a political prisoner was often to be enslaved during peacetime. Eg. China, Soviet Union
One man’s slave is another man’s forced labourer is yet another man’s Prisoner of War.
I’d say that to be a slave you need to be able to be bought and sold and probably need to be in private ownership. Though I guess State Entities could get into the property market and buy and sell slaves for particular purposes.
I imagine the Roman slave supply was actually the Legions who sold captured slaves on the open market either for their own funding or their leaders’ or for Rome.
The more nuanced forms of slavery, where it was incorporated into the economic and social culture, was from a time when human labour had a high value in pre-industrial societies.
Slaves were not bought and sold in markets by the Nazis or Japanese, the Soviets or the Chinese. They were ‘owned’ by the state and put to work in state enterprises. Moreover the value of their labour was not high because there was a abundant supply from occupied countries and therefore little effort was made to avoid death.
I would guess that the incidence of slavery in human history is probably related to two factors: large scale war involving the capture of many prisoners and occupied territories and technology. The machines of the industrial revolution changed the pattern of employment towards centralised production and a requirement for a range of technical skills rather the manual labour required by agriculture.
I would suggest the world wars of the twentieth century caused a significant peak in an otherwise steady decline of slavery that came about as industrialisation spread around the world.