There is more to the position than that, but that's pretty much as far as I get with some folks before they toss Romans 13 into the conversation as a rebuttal. After all, the claim goes, governments are all appointed by God to keep the peace. Therefore, our armed forces are just doing God's work, which means Christians obviously can and perhaps even SHOULD participate.
Here is the relevant part of Romans 13 (from the New Revised Standard Version, found on www.crosswalk.com):
Now, we can ask ourselves what this text actually does say, and then also what it does NOT say. First of all, historical context. The writer is Paul the Apostle, a Jewish Rabbi who has come to believe that Jesus is the Jewish Messiah. Both Paul and his readers (who are Christians in Rome) live in and under the authority of the Roman Empire. Nero was likely the emperor at the time -- not a big fan of Christians, to say the least. The Christians in Rome seem to be a group made up of Jewish and non-Jewish believers in Jesus.
1 Let every person be subject to the governing authorities; for there is no authority except from God, and those authorities that exist have been instituted by God. 2 Therefore whoever resists authority resists what God has appointed, and those who resist will incur judgment. 3 For rulers are not a terror to good conduct, but to bad. Do you wish to have no fear of the authority? Then do what is good, and you will receive its approval; 4 for it is God's servant for your good. But if you do what is wrong, you should be afraid, for the authority does not bear the sword in vain! It is the servant of God to execute wrath on the wrongdoer. 5 Therefore one must be subject, not only because of wrath but also because of conscience. 6 For the same reason you also pay taxes, for the authorities are God's servants, busy with this very thing.7 Pay to all what is due them—taxes to whom taxes are due, revenue to whom revenue is due, respect to whom respect is due, honor to whom honor is due.
Paul's theology of government seems to me based in the Jewish scriptures, particularly in the stories of the later history of the Jewish kingdoms (north and south) found in Kings and Chronicles and in the prophetic works that correspond to those events. To summarize, God is in charge of all of these kingdoms and/or governments. He moves them around like pieces on a chess board to accomplish his own purposes, of which humans are not necessarily aware.
Though there had been a short period of time, during the reign of King David, in which Israel had truly been a theocracy, this was not the norm. In fact, even during David's son's reign (Solomon), it seems clear that God was not being relied on for the security of the kingdom. Ask yourself this: how many wives and/or concubines did Solomon have? Answer: 1 Kings 11:3 -- "Among his wives were seven hundred princesses and three hundred concubines." Wow! This guy had at least (!) 1k women at his disposal! "At least," because the text says "among his wives"! We don't know how many more there were, but the 1k women were "among" the total number! I'm impressed!
Now, as yourself this question: how many children did Solomon have?
Go ahead, ask. Search it out in Kings and Chronicles. I'm waiting. Ok, times' up. Answer: 1 (ONE, as in A WHOLE NUMBER THAT IS LESS THAN TWO AND MORE THAN ZERO). Yes, only one.
How in the world did that happen? Well, it seems to me there are a couple of possibilities. One, there was something physically wrong with the man. After all, that one son, Rehoboam, was advised to prove to the people that his little finger was thicker than his father's loins! But as Freud is purported to have said, sometimes a cigar is just a cigar -- and a metaphor is just a metaphor.
A second possibility is that Solomon had more children that weren't mentioned in the text -- perhaps because Solomon had them killed. At least one of the "foreign gods" Solomon worshiped with his wives ("Molech": see 1 Kings 11 and 2 Kings 23:10) demanded child sacrifice. It could be that Solomon had had other children that he sacrificed. On the other hand, if that were the case, why wouldn't the text mention this in the process of listing Solomon's sins at the end of his life? It doesn't appear that the text is trying to put a positive spin on Solomon and sweep his sins out of sight under the rug (Chronicles is guilty of this, but not Kings).
The third possibility is this: Solomon was not a hedonist, and these marriages were all political alliances. The security of the nation under Solomon was not in the protection of God, but in the marriage-alliances Solomon had made with all the nations around them. Because of Solomon's marriage-alliances, he had also worshiped the gods of all of his wives, and he actually built shrines for them all. (Trivia question: how many temples did Solomon build? Answer: more than one, and perhaps as many as 1000 -- for each of the gods of each of his wives and concubines.) Because he worshiped all those other gods, the kingdom was split after his death and divided between his own son, Rehoboam, and a rival, Jeroboam. This was God's action.
Further, later in the story God actually turns against the Israelite kingdoms and brings the armies of foreign nations against them to defeat them. Pieces on the chessboard.
The point is that for Paul the Apostle, though there had been that brief moment in Israelite history in which God had ruled through David, that was long past and irrelevant to his own situation under the Romans. It hadn't worked out well anyway! And that Maccabean period? Well, again, it had ended badly, and besides that, Jesus had said things like "My kingdom is not of this world."
So, in Romans 13, Paul is affirming the truth that God is ultimately in charge, and that he uses world governments to keep relative peace in the world. Romans 13 is NOT a call for Christians to get involved! In fact, Christian involvement in the Roman government could not even be on Paul's radar screen (had he had one)! For Paul and the early Christians, God has put Rome in charge, and this is NOT an indication that God is on the side of the Roman gods, nor that the Roman government is in any sense "Christian." It is merely an indication that God is using the Romans as he has always used human governments, and Christians have nothing to fear so long as they avoid committing crimes.
On the other hand, we Christians do generally recognize that there is a time when we would be forced to invoke Peter's statement that "We must obey God rather than any human authority." But here's how I sometimes hear this one used: Christians MUST be willing to kill on behalf of our government if the government says kill.
So, if someone had been drafted and sent to Viet Nam back in the 60s or 70s, that person would have been obligated to kill, and it would have been godly to do so.
But, by the same logic, Nazi soldiers in WWII were just "obeying God" because they were "obeying orders." Those in charge of the extermination camps tried to use the "Nuremberg Defense": "we were just obeying orders." It didn't work -- they were held to have been morally responsible. By that logic Iraqi soldiers are on God's side now, and American soldiers are fighting against God because we deposed a ruler who had been set up by God. Etc., etc. Oh -- and that "American Revolution"? Uh oh. Now we're in trouble. We opposed a government set up by God. Shoot. I hate it when that happens.
You see, in Paul's situation under the Roman Empire, his words are certainly true: in essence, Christians are to stay out of the way of what God is doing with the Empire and it's power. That's it. But our situation is different, and perhaps more complicated, since we do have some element of voice in our government. Paul and the early church didn't.
Final note: Paul says "give honor to whom honor is due." I wonder if that's a blanket statement for us always to honor those in power, or if it means we have to discern who is, actually, due "honor." Of course, this statement echoes Jesus' statement about giving "to Caesar that which is Caesar's." A quick note about that story: it's one of the really great jokes of the New Testament. Jesus is talking to a bunch of Jewish scholars who are trying to entrap him, and they get trapped in their own false logic. Any Jew worthy of that title should have known that God is creator and that it all really belongs to him. Jesus threw a feint -- a "false punch" -- by pointing to the face on the coin. They went for it, and Jesus walks away without a scratch. THAT'S FUNNY!
To summarize: I don't think anyone can legitimately use Romans 13 to justify Christian participation in any kind of killing. It simply says "God will do what God will do with governments, so stay out of their way." We must always remember that we live by a different standard than the world, and that sometimes "We must obey God rather than any human authority."