Here are some challenging questions/dilemmas for anyone who believes Christians should not be pacifists.

1.) Suppose there is a Christian in England and suppose there is a Christian in France. Now suppose that these two countries are at war, and both these Christians are in the respective militaries. The one Christian will be fighting on one side and the other one against him. They might kill each other. Interesting. Can both countries be right at the same time?

2.) Now suppose a Christian lives in a country where the government makes it illegal to be a Christian and starts killing Christians and burning churches. Now suppose this Christian is in the military or the police force and is told to carry out this killing. If a Christian can participate in a millitary or police force without questioning the morality of anything, then he should go ahead and kill the other Christians. In fact, if his orders are to kill all Christians, he would have to go ahead and kill himself also.
There is only one scenario where he wouldn't have to carry out the order. If he didn't do everything he was ordered to do, but only those things which he agreed with, then he wouldn't kill the Christians. But then he isn't a soldier. No country wants a soldier with a mind of his own. Every country wants a soldier who is a pawn, who will do exactly as told. Suppose a soldier is in Irac, and is told to fly over a certain building and drop a bomb. He is in no position to ask: who lives there? What did they do to us? Did you evacuate the building? Are there civilians? etc. He must comply.

You can't have it both ways. Either a Christian in the army would have to follow the orders of his superiors without questioning them, or he would follow the Bible's commands and moral judement in all situations. So either he would kill the Christians in the above example or it would be impossible for him to be a soldier at all (if he is not following the orders of the authorities but of the Bible).