> ruling based on strength was considered the natural order.
Indeed. Athens vanquished Myteline in the Peloponnesian war and offered just this as justification. After Thuycidides, doubtless this was an increasingly bankrupt claim and so had to become more subtle.
> ... Roman's weren't racist, they were.
Were they? At base the poor modern idea of "race" is probably best rendered scientifically as "phenotype". In the classical world, new peoples could be still created (we call this ethnogenesis today -- we can read about it from classical sources). It was not larded with pseudo-Darwinistic conceits; some peoples spoke differently, appeared differently, had different customs, worshiped different gods (which Romans were keen to either adopt or accept as the same god by a different name, i.e. syncretism).
To your point, "might makes right" -- that's all there was too it to them. It is a little more complex though, in that the Romans felt themselves inferior to the Greeks culturally (probably also the Etruscans, who were in turn influenced by Greek colonists), and so had to contrive the idea of the genius of their ethnos being one of administration (probably the genesis of this article's conceit). I'd be much more interested in an article about how to be a better Greek in a Roman world, candidly.
> Almost an ancient version of realpolitik.
Well, exactly that, though it would have simply been regarded nakedly as Politik. I love the ancient world because they saw some things so much more clearly than we do (without the subtlety, probably because they still had to worry about the corn supply -- erst kommt das Fressen, dann kommt die Moral). That we care not today for the ancient world serves us poorly, and only delivers us as prey to today's howlers.
> human rights were not derived from an enlightenment idea of human quality
In point of fact, the Enlightenment draws its origins to early humanists dredging up the wreckage of the ancient world. Timely controversies about whatever-the-fuck Joe Rogan said are drowned in a benighted sea of censorship lust, ignorant of the humanists' salvage and our universal birth-right. Give me your debunking of Voltaire or explain where Poggio went wrong, and I'll give you my earnest attention.
If you want to speak of Greek rationalizations for slavery, you may as well conduct a census of the Realpolitik of the region. A student of the hotchpotch of ancient tongues will be confronted with a hotchpotch of causes ranging from contesting neighbors to debt-bondage. Little different in Latium.
> However, Aristotle's argument that inferior humans benefit from slavery did not use race to define the inferior.
Right, and when you read the ethics and see what friendship is about -- you understand they saw relationships as being about mutual benefit between the superior and inferior. A couple millennia of Christianity haven't removed the question in the back of our minds, "but what's in it for me?" Again, they saw some things so much more clearly and without bullshit than we do today, and yet, yes, Aristotles' ideas about cuttlefish reproduction are sources of great mirth (and stuff like Plato's Timaeus great fodder for a thinking-man's Art Bell episode).
Again, at the risk of over-simplifying, slavery in the ancient world (and throughout history) was a quite different institution than what we think of today, thanks to the U.S.'s constitutional pusillanimity (wake me up when the U.K. abolishes the monarchy -- that's apparently harder to do than abolishing slavery; I guess it's easier to become a subject than a citizen under some empires).