Joining a war, as a christian, puts a person in the position of possibly murdering someone. Murder is like judgment, ya don't wanna do it. To murder someone is to pass a judgment, like they are not good enough to live or god didnt want them saved anyway.
Personally, i wouldn't want to be in that position.
The OT wars were won or lost by the discretion of Yahweh. He caused israel to win or lose depending on "sin in the camp". Today there is much sin in the camp. God will do what god will do and there is no diverting it. God raises up kings, presidents and nations, they are all chess pieces in his hands.
If I, as a christian got killed in a war, that would be like committing suicide, the work of god in my life would be over and i would fall short of what He wanted to do in my life.
Some areas of this sinful, carnal world we are seperate from and i believe war is one of them, police, politics are a couple of others.