Davis, Curtis C. and Dvornik, Francis
"Origins of Intelligence Services: The Ancient Near East, Persia, Greece, Rome, Byzantium, the Arab Muslim Empires, the Mongol Empire, China, Muscovy,"
Naval War College Review: Vol. 28
, Article 11.
Available at: http://digital-commons.usnwc.edu/nwc-review/vol28/iss5/11