The deeper meaning or theme of "War" is that there is nothing that can compensate people for the lost lives of their children. To the simple people riding on the train in Italy, each one with a son or sons either at the front or already killed in World War I, love of country or the cause of the greater good can not begin to equal the losses they have suffered.
Pirandello personalizes war. It is not about abstractions like king or country or patriotism but about how the death of beloved children cracks and overwhelms the survivors.
The people on the train quarrel about who suffers the most: the parents who lose an only son, or those who lose one son and still have one left. As one passenger states, love for one's children knows no bounds:
Paternal love is not like bread that can be broken into pieces and split amongst the children in equal shares. A father gives all his love to each one of his children without discrimination . . .
One man speaks up and articulates the "official" line: how can we think only of our own loss when our children are giving up their lives for the love of their country—and because they want to do so? He says he does not grieve his son because his son willingly sacrificed his life for his country. But when a woman on the train probes him, asking if his son is really dead, the reality hits him and he breaks into sobs. In the end, the idea that these young men are doing a good thing for a good cause is broken down: the war is meaningless to ordinary people except in terms of the great grief and pain it brings.
The story speaks to the deep trauma of World War I, a great bloodbath with a vast loss of life. The story was written not long after that war ended and calls into question what its purpose was. Like many others, Pirandello felt the great waste and meaninglessness of that conflict.
Two companion pieces that similarly reflect the sense of trauma the war brought are Katherine Mansfield's "The Fly," which is also about a father grieving the loss of his son in World War I, and Wilfred Owen's poem "Dulce et Decorum Est," which shows the suffering on the battlefield—and like "War," attempts to expose the lies told to justify war. However, "War" is a gentler, if still deeply moving story, as it doesn't force the reader to look directly into the face of war's cruelty. Here, we see it refracted through the suffering of bystanders.