pster wrote:Is there an implied copula (is) as I have indicated? Seems there has to be or else the subordinate lacks a verb.
But this could be said of the main sentence as well. There is no verb in either clause.
What connects them is ἔστιν with τοῦτ᾽ referring to the entire subordinate clause.
I would also read the subordinate as relative clause with ὅτι representing ὅ τι.
This is supported by the Perseus translation (thanks for the link, btw):
What is really educational and beneficial to students of history is the clear view of the causes
of events, and the consequent power of choosing the better policy in a particular case.