That's the theory. In practice, people really do just follow orders, the handful that stands up gets mowed down quite easily. Look at the large number of casualties in Iraq and Afghanistan (and Vietnam and a dozen other places) and what came of the numerous war crimes committed. You don't need 20/20 to know that when you enlist the chances that you are going to be part of something bad are pretty high.

Have a read:

https://en.wikipedia.org/wiki/United_States_war_crimes