I am seeking a reference to an article or book by Bernard Knox
discussing the defeat of Athens in the Peloponnesian War.  Several
quotations are "Athens' tragic destiny takes a larger sweep, in
proportion as her stake in the greatest of all - freedom or empire." 
"defection of Athens from the old, glorious ideal of the union of
Hellas against the outer darkness of barbarism."

Hello perses,

I have not been able to find your second citation, so am putting this
in as a comment rather than as an answer.

The first citation you provide is actually from Francis M. Cornford's
"Thucydides Mythistoricus" page 153, as you can see in the etext from
Perseus collection at Tufts:
http://www.perseus.tufts.edu/Thucydides/Cornford/CCh.9.html

The publication details for the original 1907 edition, as given in the
British Library catalogue are:
Thucydides Mythistoricus, by Francis Macdonald CORNFORD. 
Edward Arnold: London, 1907. 8o. pp. xvi. 252. 
http://blpc.bl.uk/

A new edition of this work is due sometime this month from Duckworth Publishers
http://www.internationalpubmarket.com/clients/duc/Books/BookDetail.aspx?productID=76639
Thucydides Mythistoricus
F.M. Cornford , Simon Hornblower
Paper: List Price:  $25.95   Your Price: $19.46
ISBN: 1853996653   Available: January 2004