The two are completely against each other from the start. Ethics, motivation based on ideas of right or wrong. Who is to decide what is right or wrong? Ethics cannot be taken to such a large and almighty scale as war, because in their own minds each side is right, and therefore their own ethics will prevail. Then, who has the right ethics? Are there right ethics? For example, as I tend to lose my way and confuse people when I'm typing.
When rifle using hunters of the American settlers first began to employ their skills on the battlefield, they formed what has now become the modern day sniper. At the time, it was against the "rules of engagement" (ethics) for them to shoot a man on the toilet, unarmed, non combatent, etc. These days (and I can tell you this from personal experience) A sniper will shoot at any man who has/will/may have possed a threat. When did the rules change? Why did the rules change? Surely what's right and what is wrong does not change? Or is it far more colourful than the black and white I chose to see?
Another example of change, would be the Samurai or ancient Japan. Now I'm sure everyone here would have a slight idea of what I meant when I mention their honor. The way a Samurai lived was based around teachings of peace, yet who would believe it when they are known to be some of the greatest warriors ever seen? The point I am making, is a Samurai's ethical code was something stronger than anything we would see today, and this was sown to the way things were in feudal Japan. Yet, amidst the honor and all that, Ninjitsu Assassins were employed. A Samurai cannot attack unarmed enemies with his weapon, must only seek out other Samurai on the battlefield, and must face his enemy. A Ninja on the other hand, can hide, attack anyone he needs, poison, and generally corrupt the normal rules of war. Two forms of combat used by the same peoples at the same point in history. Both completely different ethically.
This rant can lead me to one conclusion, ethics, will never walk hand in hand with war, this is because ethics is forever changing in the face of war. Ethics only go as far as we want them to, because they are self placed restraints, it's easy to speak of ethics in times of peace. But in times of war we must forget those words.

war is, by it's very nature, inhuman and brutal. it is the exercise of killing on a mass scale, with sociopathic disregard for life, often for purely selfish reasons for those who declare such actions "neccesary".

but those who fight in wars are still human, and do not like it any more than the victims. for these, war has, or is supposed to have, a code of conduct that is ideally supposed to lessen the horrors of it (as if that were in any way possible), and ease the consciences of those called upon to fight. some of these rules are written, such as the Geneva Convention, and some are implied, such as the use of serrated bayonets.

politicians start wars, but seldom look at the consequences of their actions. that is something the victims and the soldiers live with every day, long after the shelling stops and the bombs are quiet.

it would be a better world, if those who declare war had to put on a uniform, and go to the front lines themselves. if they had to see it first-hand, maybe they wouldn't be so eager to deem it "neccesary"