Quite true cocoa Aristotle did construct a defence of slavery; his problem was that the "wrong sort of people" were being made slaves. His argument presents a hierarchy of rationality, with (roughly speaking) adult males at the top, then women and children, then slaves. The problem was to allocate the "right" people to the slavery class—those whose powers of reasoning were so restricted they required someone else to set their life-goals for them and ensure they met them as far as possible. Reason here is not just means-ends reasoning, which people lower down the hierarchy do have, but also enables the sort of rational reflection on theories and experience, conceptual analysis, and abstract thinking that allows a person to discern what is good or bad overall. This distinction is already implicit in the argumentation in Plato's Republic concerning the tripartite structure of the soul and the parallel tripartion of the ideal society. Aristotle is thinking in political terms only, however, and regards slaves as "living instruments" for use by their superiors.
As you say, general enslavement followed defeat in war, and the horrors of it were well-known.
There's a fragment of a Greek poet, Hipponax (probably), that focusses vividly on slavery, as part of a curse, when the poet imagines the man who betrayed him being shipwrecked and enslaved (Thracians, who lived in northern Greece, were considered uncivilized). Here is my translation:
'…battered by the waves;
Thracians with those top-knots of theirs
up in Salmudessos
will take him, naked, to endure
evil upon evil
eating the crust of slavery—
shivering, chilled from the brine,
seaweed covering him, his teeth
chattering, powerless, lying like
a dog on his face there
right where the surf breaks…
All this I’d like to see:
the one who wronged me, the one who
ground oaths under his heel—
he who was once my friend.'
Enslavement continued into the Hellenistic period, with Alexander's expansion of Greek control and civilization. During this time a genre of anecdotes became especially popular: a philosopher is enslaved, and then meets the conquering leader. For example, one philosopher was asked whether he was all right and whether he had lost anything. He replied, 'I have all my goods with me', meaning that he did not value possessions, or even being free, as good, but only virtue, knowledge, or philosophy, or some combination of these.
It's a variation on the genre of anecdotes about philosophers meeting kings or other leaders. An early example of this genre (entirely apocryphal, of course!) was between Diogenes the Cynic and Alexander. Alexander asked Diogenes whether he could do anything for him (this alleged meeting would have taken place in the market square, where Diogenes always hung out). Diogenes replied, 'Yes, please move, you're blocking the sun'.
The point is that philosophers did not start arguing that slavery is wrong, despite its evils; they thought about how a philosopher, or a wise man, ought to behave if enslaved. A lot of moral philosophy was done in the Hellenistic period by discussing what a wise man ought to do or not do, and this is one more example of that trend.
Seneca says that the Roman Senate had debated ordering slaves to wear distinctive clothing; there was a lot of pearl-clutching going on about how slaves had begun wearing better clothing than free men, being carried in litters, etc. (slaves could legally accumulate wealth). It was realized in time that this would only alert slaves to how many of them there were! I am not sure if this report is substantiated by other evidence.