There is an excellent article in Vol 33, Issue 1 of Themelios, in which Kirk MacGregor outlines the evidence from the ante-Nicene Fathers on their stance regarding Christians, violence, and soldiers. He outlines three facts in particular:
1. That until 174 AD, no Christians served in the military or assumed government office
2. From 174 to the Edict of Milan (313), the church treated Christians in such roles, or who had left such roles, with "great suspicion"
3. That the position of the church in this period was derived from a theory of nonviolence based on the preaching of Jesus.
Makes for a good read.