My opinion is, I think there can be a just war if you are a Christian. the just war theory itself was made for those Christians in doubt. Jesus (a Christian's role model ) himself proclaims that he will destroy the earth during his second coming. So when Jesus himself is fighting to destroy evil (one of the just war conditions), that would be a just war. Many may say, Jesus also said to 'love your enemies', but who was he talking to? He was talking to the ordinary people, whose enemies may be a person who they hate or did wrong to them. Wars are declared by leaders not ordinary people. The whole reason there are governments and groups of power is to make sure that a country is safe and improves to be it's best. Jesus did not say ' love your enemies' to leaders and rulers of countries. And remember, Jesus also said ' blessed are the peacemakers '. If a country was having a civil war, would it not be right to support the good side and destroy the evil going on and establish peace? I want to conclude with, ' the evil which results, is much worse than the evil of war'. Thank You for reading (just to say, don't be deceived by the just war theory, by thinking that a just war is right. but a just war theory is permissible, as it is still an evil act, but for the right cause)
Well to start of being a christian is a very broad term especially because of the extensive amounts of christian denominations that vary from catholic to protestant to Pentecostal and so forth. Well to counter your argument of Jesus coming to destroy the earth on his second coming you must understand a term in Christianity called the judgement in protestant Christianity which believes in the second coming or the day of judgement is a process in which those saved by faith are saved and those who rejected the grace given by Jesus death in the cross are left in this world.
Now to address the main issue of what is the meaning of a war if you are a christian we have to understand what war is. war is defined as a state of armed conflict between different nations or states or different groups within a nation or state. So war is basically an argument between humans, in which humans want to reach to a conclusion through armed conflict. This brings up one of Christianities main principles we are sinful even when we are in our mothers womb and therefore we are inclined to do evil.
So in conclusion War is just an indirect consequence of sin, so as a christian a war should only be a reminder of the atrocities of sin, and call to revival of spirit and to allow God to do its fair judgement.