I think there are a range of reasons why someone might come to a position of conscientious objection after joining the military. Perhaps someone joined the reserves for other reasons (e.g., college tuition: this is one of the biggest recruiting tools on college campuses, though it is not always as good of a deal as it is made to seem) and then was sent over. I know it sounds unbelievable, but particularly someone who signs up for JROTC in high school or ROTC in college might not yet have the life experience and maturity to understand the depth of what they are doing. They truly may not realize what war is all about until they see it, for example. Video games and movies certainly are not giving them the truth about war. Many mature adults don’t truly realize what war is, so how could an 18 year old?

I think another sad reason is that most churches do at best an awful job at sharing the Gospel message and teaching Christians about a valid Christian response to war and violence, and at worst send a thoroughly nationalistic (not Christian) message that engaging in war is the height of Christian sacrifice. Because of this, at least some Christians may join the military and get sent to a war zone, before they start to reflect on their faith and what it teaches about participation in war and killing. It is almost impossible to make a mature, moral decision without proper information.

A good resource on this topic is:
http://www.catholicpeacefellowship.org/index.asp