Shall thinks themselves accursed they were not here;
And hold their manhoods cheap, whiles any speaks
That fought with me upon St. Crispin's Day.
---Henry The Fifth, King Henry the Fifth. Act 4, Scene 3
As far as I can tell this speech may be the origin of the trope in war films - and Battlestar Galactica - of drawing the line where the guys who don't have the stomach for it are free to leave and not take part. Peer group pressure dictates that nobody ever does.