There are some unfortunate circumstances where one's liberty can be preserved only at the expense of someone else's, and where the citizen can be perfectly free only if the slave is completely enslaved. Such was the situation in Sparta.
Gist of Idea
Sometimes full liberty is only possible at the expense of some complete enslavement
Jean-Jacques Rousseau (The Social Contract (tr Cress) , III.15)
Rousseau wrote just before the moment when it was seen that slavery in European empires might be abolished, but he was not in the forefront of thought on this one. Greek philosophy would probably never have happened without slavery.
Rousseau,Jean-Jacques: 'The Basic Political Writings', ed/tr. Cress,Donald A. [Hackett 1987], p.199