Luk 3:14 And the soldiers likewise demanded of him, saying, And what shall we do? And he said unto them, Do violence to no man, neither accuse any falsely; and be content with your wages.
In Luke 3, the text quotes significant messages from the ministry of John the Baptist. John’s ministry was to “prepare the way for the Lord”. The text records his response to a few soldiers about how they should behave. We need to remember the Jews had special exemption from the Roman government for forced servitude in the military. These soldiers speaking to John were most likely volunteer Jewish auxiliary troops for the Roman Empire. Rome, contrary to modern depictions, did not have a large Roman military presence in Israel at the time of Christ.
The word for “violence” is sometimes translated as “extort money from”, but I think that translation fits “violence” (or “intimidation”) better. The context could go either way. The immediate context does deal with money “tax collectors not taxing more than authorized” and “being content with wages”, the overall message is about the end of the world. The entire message is in a world where the Romans have put down several prophets similar to John the Baptist for similar teachings. While some Roman soldiers (Jewish Auxiliary troops) might have engaged in theft, the accounts of the time focus on Roman oppression from the established powers. The soldiers were often used to suppress Jewish revolts. Due to this persecution and the soldier’s allegiance to Caesar, Jewish auxiliaries were seen as traitors. The same goes with the tax collectors. This is probably what compelled the soldiers to ask John in the first place. John the Baptist was held in high regard and they were looking for his perspective on the matter.
The same word for “violence” is used in Polybius’ Histories described a terrible tyrant:
Summoning their sons or husbands on absurd pretexts he intimidated them, and on the whole behaved in a most outrageous and lawless manner.
In any case, John the Baptist did not seem to think of Jewish auxiliary troops as traitors. John did not oppose military service, as so many Pharisees of the day. John the Baptist did tell them to do no harm and to be content with their wages. This is a good lesson for both American police and military personnel: the police which have a stigma of abusing innocents, and the Military which is paid exceptionally well (better than most people realize).
reality is not optional 
If John the Baptist, Jesus and Peter were against military service and all that entails, all three of them had their chance to directly address it when they had interaction with Soldiers/Centurions. Such as when John was asked what must I do he answered them, not do no harm, which is the role of the military, but as the majority of translations state do not extort or extort money with violence. When Jesus healed the servant of the Centurion he said he had not seen so great a faith in all of Israel. Peter went to preach the Gospel to Cornelius, a Centurion who sent for him at Joppa after an angel of the Lord told him too. The angel said it was a fulfillment of God’s promise to Abraham that all the nations will be blessed by his seed. Peter was also instructed by the Holy Spirit to go with the men sent by Cornelius whereby he shall tell him what he must do to be saved. Peter preached the Gospel and as he was finishing the Holy Spirit fell upon Cornelius and his household. Peter exclaimed to the Jewish brethren who had gone with him, if the Holy Spirit has been given to them as it has been to us who can with hold water for Baptism. He commanded that they be baptized and he stayed with them for a few days. So in all these incidents John, Jesus and Peter at no time were the soldiers told they can not be soldiers or that being a soldier was by nature of the occupation a sin. Rather, as in Romans chapter 13 we are told the Governments are in place by God to punish the evil doers and part of the Government’s ability to do that is with the police or military. The Bible tell us thou shalt not murder, which is unlawful killing, therefore there is lawful killing, David killing Goliath and unlawful killing, David arranging for Uriah to be killed.