I see modern special forces today wearing masks as a tool of psychological warfare? Why didn't armies before wore masks during battle to have a psychological effect on their opponent? Was there an instance of a military doing this in history?
This was done by some cultures. Samurai and Aztec are two that I can think of. Some cultures didn't use masks, but used face painting for the same thing. I cannot speak too much on this subject, but this should give you a starting point for further research.