I'm writing a war story, and it's dark. However, I find that every scene turns out to be depressing because of it. Readers will be overwhelmed. Are there ways I can induce hope/shine the light in the darkness in my novel?
Some of the stories that I have read are incredibly dark, based simply on the events that occur, but did not feel that way when read. Bringing out a lighter side can be done in a number of ways:
Having characters that are hopeful or optimistic will go a long way to brightening a story (unless they are annoyingly optimistic to the point of delusion). Even when terrible things are happening, when the horror becomes an everyday reality there will be people who do not let it get to them, and can still remain light-hearted and boost the morale of others around them.
If your characters feel less downtrodden about what is happening to them, then the reader might feel a little less beleaguered when reading the story.
Even if you do not wish to introduce a character like that, every character will have stories to tell that are perhaps humorous or uplifting. The scene near the end of Saving Private Ryan where Matt Damon's character tells a story to Tom Hanks' character about the last time he saw his brothers would be a good example of that.
Even the most miserly characters will have tales from their youth of when they were happy and carefree. If the readers are needing some relief from the reality of a situation, then the characters certainly will be too. Having interactions like this are unlikely to feel out of place in slower parts of the story, and it will allow insight into the characters and their backstory.
Light at the End of the Tunnel
Similar to every character having a backstory, they will also all have a vision of the future. Maybe they have a family that they want to get home to, or plans to buy an abandoned shack at the edge of town and turning it into a pub.
Having the characters hang on to their humanity, and retaining a sliver of hope that they will pull through and survive the war, will allow them and the reader a better outlook on the situation.
This doesn't have to be the characters winning every battle, but being able to find successes in the small things even despite the losses. Perhaps the enemy managed to capture an allied checkpoint, but your characters manage to evacuate all of the civilians or retrieve a full cache of weapons before they are forced to retreat before the enemy closes in on their position.
It could even be personal successes instead of ones based on the plot or the the war, such as receiving a message from back home that their child has been born healthy and happy, or they successfully created an edible meal from their rations by combining it with some of the local flora they found whilst scouting.
So in summary, just like you are trying to find ways to make your book more bearable for readers, the characters will be looking for ways to make their own lives bearable. No one wants to feel awful, particularly in the worst situations, so they themselves will seek ways in order to lift their own spirits, and make a better situation for themselves.
You can talk to real-life veterans and see how they coped with war. One tactic is "gallows humor" or "black humor," which is seeing the humor even in grim moments (common to veterans, law enforcement officers, doctors, and first responders). The TV show MASH was essentially built on this. There are many examples on the TV Tropes page (consider yourself duly warned).
What I would do, is give the characters something to look forward to. Give them something to fight for. They need a motivation to keep going, something to comfort them in their time of need. I think by giving the characters this, you'd be able to make them have more courage and generally be happier for what is to come.
Always give characters a motivation.
You could make the story lighter by setting some scenes in a peaceful landscape, perhaps in the homes of the characters, showing who they are fighting for.
You could have the protagonists suffer defeats and setbacks and hardships in the first chapters, and start to break down, and seem on the verge of defeat. Then have chapters set in the enemy camp,and show that the enemy soldiers are also on the verge of breaking down and think that they can't go on much longer. This shows that victory or defeat are still balanced on a knife's edge and could go either way, and that in any case the war can not last much longer. This works in a relatively evenly balanced war like the American Revolutionary War or the American Civil War, but not in a war where the enemy has overwhelming superiority.
Or you could have chapters set in the distant future, a time of peace, when kids are studying the war your characters are fighting in, and trying to understand what it would be like to fight in such a terrible war. Perhaps the teacher could mention to them that they owe part of their present happiness to the outcome of the war, without specifying how the war ended since they already know that, unlike the readers or your characters.
You can lighten the darkness by contrasting cruel deeds with a few acts of kindness.