God is true, but he is also love.
These are both true statements about God's character, and I think we need to not forget, when we consider lying and whether or not it is morally acceptable to lie, that "God is love" is the most basic and important thing that we can say about God's character.
So you can mentally work through the issue of lying in wartime situations in a couple of ways.
The ninth commandment is often translated "bear false witness," so I had always thought that it was referring specifically to giving witness in court, i.e. you shall not falsely bear witness in court saying that an innocent person committed a crime in order to have them wrongfully condemned. In a society in which eyewitness testimony was basically all anyone had to go on to determine whether someone was innocent or guilty (no DNA testing), to pervert justice and cause an innocent person to be condemned by falsely bearing witness was a very heinous thing (and still is now).
So that commandment is not prohibiting white lies, wartime lies, lies to protect someone's life, that sort of thing.
And while there are other places in the Bible where truthfulness is upheld and lying is condemned, and of course truthfulness is valued and we should want to be truthful people, not deceptive people, if you look at the context of what specific kinds of deception are condemned, it usually is something that causes a person to have an unfair advantage over someone else (like the unequal weights and measures which someone else mentioned). That, or lying in order to preserve oneself from justly deserved punishment, would certainly be wrong, but you can look at those passages about lying and justifiably interpret them to be a prohibition against things that would cause harm/disadvantage to others or pervert justice or would be a selfish and fearful avoidance of a deserved punishment to oneself, that sort of thing.
So Christians should want to be people of integrity, we should not pervert justice and we should not lie to cover up something we did wrong ("No, I didn't cheat on that test"). Generally, people should be able to trust us and take us at our word.
But I think it can be argued that the Bible's prohibition of lying doesn't mean that we have to tell the truth if it will cause harm to an innocent person.
That's one way of thinking about it.
I'd also like to submit for your consideration the fact that we are living in a fallen world. In a perfect world, if there were not sin in the world and if no one ever committed evil against each other, there would never be a reason to lie that would be "for the greater good." But because we live in a world in which there are sinful actions and people do things like genocide, sometimes there are situations where two values, like being honest and being loving, clash with each other, and I am absolutely certain God is not going to condemn or look down upon someone who tells a lie in love in order to save an innocent person's life.
There's a book I would really like to recommend to you, The Hiding Place by Corrie Ten Boom, and it's about a family of Dutch people living in Nazi-occupied Holland during WWII, and they were sheltering Jews in their houses, and they were raised with a very strict upbringing to think that telling any kind of lie is always wrong, and they had to wrestle with the very same qeustions that you are wrestling with.
There is one point where Corrie's family (which owns two radios) is having to turn in their radio because the Nazis are collecting them all and no one is allowed to have one. So Corrie goes and turns in one of the two radios, and they hide the other one, and when she is asked if that is the only radio their family has, without hesitation she tells a completely brazen lie and says that it is their only radio and they don't have another one. So then later her family is having a big argument about lying and whether or not it is permissible in such situations, and Corrie says something heatedly about how it was her lie that allowed them to keep their second radio (this is important so that they can get news from the outside world and not just Nazi propaganda). And in the middle of that heated argument, her father, who is a wonderful, godly old man, says, "I'm sure whatever Corrie did, it was done in love." And that's the important thing.
There are also a couple of stories in this book about times when people in the Ten Boom family, believing that it is never okay to lie, told the truth to the Nazis and things miraculously worked out okay in the end. For example, there was a household where they had a trapdoor in the floor underneath their kitchen table, and it was covered with a rug or something, and that trapdoor led into a hole or a basement or something where they were hiding Jews. So the family was sitting around their table, and there were Jews right underneath them, and some Nazis burst in and asked, "Where are you hiding your Jews?" And a young woman of that family gave the immediate, honest answer, "Why, they're under the table," and the Nazi soldier lifted the tablecloth and saw nothing under there. And the young woman, because of the tension of the situation, burst into hysterical laughter. So as a result, the soldier thought she was lying and making fun of them, and without searching any further the soldiers got mad and stormed right out again and the Jews were completely safe (though justifiably rather shaken).
But there might also be many cases of people who told the truth for that very same reason about sheltering Jews and had a less fortunate result. If I were the one sheltering Jews from the Nazis, I would lie through my teeth and not feel a single ounce of regret.
Anyways, in these cases, where someone is protecting an innocent person's life by lying, they are preserving justice, not perverting it, and I think the good of preserving an innocent person's life far outweighs the evil of telling a white lie, and if you consider the fact that there are situations in which there are moral dilemmas to be a part of living in a fallen world and make it your principle to do whatever will result in the greatest good and whatever action is acting with the most love towards the people around you, you are not making God out to be inconsistent (it surely grieves God that there are situations where one cannot act in love and also tell the truth, but surely he is grieved less by a loving lie than by an innocent person's life being lost), and you also don't have to jump through quite so many mental hoops before reaching the conclusion that it's better to tell a lie to save someone's life than to tell the truth at the cost of their life.
Let others believe in the God who brings men to trial and judges them. I shall cling to the God who resurrects the dead.
-St. Nikolai VelimirovichMAL