Here is an absolutely fantastic essay in Books and Culture by Ron Sider, who has collected into one volume every single available piece of patristic evidence regarding how early Christians responded to war and killing. Here is his synopsis of the evidence:
Starting in the late 2nd century and then increasingly in the later 3rd century and the first decade of the 4th century, there is evidence that some Christians were serving in the Roman army—at least a few by AD 173, and a substantial number by the late 3rd and early 4th centuries. Unfortunately, our sources do not enable us to say how many.
On the other hand, there is not a single extant Christian author before Constantine who says killing or joining the military by Christians is ever legitimate. Whenever our extant texts mention killing—whether in abortion, capital punishment, or war—they always say Christians must not do that. . . Indeed, there is very little basis in the texts for describing the early Christian view as “divided and ambiguous.” There are no authors who argue that killing or joining the military is permissible for Christians. On these questions, every writer who mentions the subject takes essentially the same position. Some pre-Constantinian Christian writers say more about these topics than others. Some do not discuss them at all. But to conclude from this relative silence or paucity of some surviving texts that other writers disagreed with the extant texts would be sheer speculation. The texts we have do not reflect any substantial disagreement. Every extant Christian statement on killing and war up until the time of Constantine says Christians must not kill, even in war.
For an additional treatment of the evidence, see Kalantzis’ “Caesar and the Lamb“.