What would it take for you to willingly sacrifice your life? Or to take someone else's life? In wars, people do both. What does it take to convince large numbers of people that dying and killing are worthwhile?

One might think it only requires someone giving an order; but, there are many cases of people refusing to fight - many men during WWI and WWII, for instance.

When called upon to fight and die for their country, most people comply; but sometimes they walk away... or join the other side.
Determining why is, in part, a problem of ideology - of the way they think and what they think is right.