Why did the american officers tell the volunteers not to fire "until you see the whites of their eyes"?

the mongolian invasion of china and central asia resulted in a disruption of the silk road as the chinese garrisons lost support from the chinese kingdoms which fell under the mongols. however, as the mongols established a formal dynasty in china, the successors of genghis khan restored the garrisons along the silk road allowing the safe travel caravans again.

500 books can be produced by one man and a printing press within 5 months, while a copyist can only produce one in the same amount of time