763Athens

A: Don’t try to form the possessive of “Athens.”
B (visibly shaking): It ends in an S, so it’s Athens’… But it’s singular, so it’s Athens’s… But ancient names ending in S form a possessive with a simple apostrophe, like Socrates’ students or Xerxes’ army, so its Athens’… But the ancient form, Athenai, doesn’t end in an S, so it’s Athens’s… But the ancient name is plural in form, so its Athens’… But the -enai ending is actually pre-Greek, so who knows whether it was originally a plural or not…
A: That way lies madness. Just rewrite your sentence.