I thought that in history, Shin as the one who proposed to only send 200,000 men despite Ou Sen stating that something like 600,000 would be required, but Sei decided to trust Shin and sent him to conquer Chu.
Also, I don't remember reading that any traitors were involved. Shins Army was ambushed upon moving into Chu lands, and were able to push Shin back due to the intelligence of the commanding General in charge of the Chu armies.