Men are seldom born brave but ...

Men are seldom born brave but they acquire courage through training and discipline - a handful of men inured to war proceed to certain victory; while on the contrary numerous armies of raw and undisciplined troops are but multitudes of men dragged to the slaughter.
 Publius Flavius Vegetius Renatus

Quotes from the same author

No great dependence is to be placed on the eagerness of young soldiers for action...fighting is agreeable to those who are strangers to it.
 Publius Flavius Vegetius Renatus