There was a lot of debate in the early church as to whether military service was right or not.
According to Tertullian there was the "Thundering Legion" in 173 was saved by Christian soldiers who prayed, and brought a thunderstorm filling the water barrels, and caused the barbarians to flee.
Origen of Alexandria argues that Christians should not not say that the power or prayer was enough.
The Canons of Hippolytus from ~340 state that
-No Christian should voluntarilt brcome a soldier
-If he becomes a soldier he should not shed blood... really
-If he has he should stay away from the sacred mysteries until he has done penance
St. Augustine, after the sacking of Rome in 410 began to lay down the theory of "Just War".
Using Christianity to officialy sanction war began in the 600's when the Persians took Jerusalem and the relic of the true cross. Heraclius Emperor of the Eastern Empire in effect led the first crusade against the Zorostrian Persians, and successfuly returned the relic to Jerusalem in 630 only to lose the city to the Muslims in 632. In the West Christian military orders didn't come up until the Muslims reached Iberia (Spain & Portugal).