The controlling idea of the story "War" is how the people cannot control some events around them, no matter how much they may believe they can. Humans tend to believe they are superior and control anything and everything they set their minds to; however, many events in this world are beyond our control. For example, the people on the train pretend they are in control, but are living an illusion. They choose to glaze over the war and its atrocities and try to rationalize involvement in the war, etc. One man whose son has died tries to find valor in his son's death, but at the end, he breaks down once it truly hits him that his son is dead and he has been living an illusion by telling himself that his son died doing what he loved, etc. Masterplots on eNotes state that:
Luigi Pirandello's characters are prisoners of their own subjectivity and their own lack of imagination. They are morally featureless. However, their stale words and feeble efforts to communicate, coming from their boredom and trepidation, reveal a genuinely human need. They must convince themselves of their own intrinsic worth to alleviate their desperation.
I like what the last person wrote, and I think you could tie her response to Plato's Cave theory. According to Plato, everyone has a "film" over their eyes and once they get away from that film they fight it and it takes a while to see it. I suggest looking up Plato's Cave theory and see just how it ties to this story. Hope this helps.