Originally Posted by T. A. Gardner

PFM! PFM! Actually, for this one I used

The Red Army Order of Battle in the Great Patriotic War by Poirier and Conner
Companion to Colosus Reborn Glantz
and
Nomohan: Japanese-Soviet Tactical Combat 1939 by Drea

It isn't too hard when you have amassed a library with literally thousands of titles and have been studying this stuff for decades. All I have to do is pull a few books off the shelves and volia! Instant massive data.