I believe it is that war makes people commit bad deeds such as kill people. I am comparing it to Saving Private Ryan. I need help please. Thanks for the help.
Theme is not always easy to narrow down and you can often come up with more than one theme. Theme is just the controlling or central idea of a story. You could work with the idea you stated in your question and you could also consider something like 'the people stuck in the middle of a war and how they cope with violence, tragedy, and loss' as a theme.
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zward03 is right, all my historical fiction is about how people deal with being stuck in extreme situations they don't fully understand. Consider how each character adapts. They start by falling back on ideology but that doesn't work in the situation they find themselves in. In one sense it is the youngest character who adapts best. Have fun with it.