Every major issue in life is complex and even wise and good people take different views of them because there is so much to see. Here's my present sense of things. Our sinfulness affects our reasoning (Romans 1:21, Colossians 1:21) and it’s no surprise that we end up giving more weight to one argument than another or playing down some critical point against the view we incline to. We still know that 2+2=4. It isn’t that we have become incapable of using rational faculties but our vested interests blind us to truths we care not to see. (Compare John 5:44.)
But our sin is not the whole story. If Paul can speak of the Jewish Torah and say it is holy, righteous, spiritual and good and in the same place speak of it as a torah of sin and death (Romans 7:12-14,23 and 8:2) we need to recognize that there’s more than one perspective to a complex reality. In light of Romans 13 we might think that governments are all pro-God but when we bear in mind that Rome was the fourth beast of Daniel 7 and the 4th kingdom of Daniel 2 we know we have a complex situation on our hands.
But God’s self-revelation in Jesus Christ through the Spirit speaks an eternal judgment against our warlike ways. When I say "our" I mean the human family and no one segment of it or individual in it. So when Paul says the works of the flesh include divisions, factions, enmities and strivings he goes on to say that those who are Christ’s have crucified all these (Galatians 5:19-24). If giving ourselves to Christ means we crucify all these things then it must mean that Christ indeed came to expose and condemn them all. Since the Christian’s conduct is not mere morality but the commitment to Christ-likeness then he or she would have to be opposed to war. And beyond the individual it would mean that the whole church, as the body of Christ, living out and rehearsing the whole gospel about the Christ would be opposed to war.
We may not be able to settle what it is precisely that James 4:1-2 is getting at but there’s no mistaking the central thrust. War, or what is warlike, arises out of our selfish drives. [I’m one of those that think James is writing to Christian and non-Christian Jews so that his words would take in the rich oppressors of chapter 5 as well as those who wear the name of Christ.]
I would have thought that the business of the lives of members of the body of Christ would be to proclaim the reign of God as the reconciling of the world to himself and consequently to one another. I don’t see how a Christian can pursue that goal in taking (or trying to take) another person’s life in war.
Since the day we plunged ourselves into the darkness of rebellion against God we’ve been warlike and warring, at the individual, national and international levels. God came in Christ and continues in the body of Christ to condemn all that and more. It appears to me that Christians, if they are true to their calling, should do the same in life and teaching.