On the scale of a full army, half is extremely heavy even; my understanding is that somewhere around a quarter or so is what it takes to defeat an army.

Imagine being in a group of say 10 men, marching towards an enemy position. You get halfway there, but two of your friends are hit by arrows and go down screaming. Sure, you might reason that by the time you make it all the way you'll only lose another two men, but one of those two might be you. At what point do you think "Well heck, the next one arrow's going to hit me!" and decide that you're not walking into that?