I had been forewarned that this was not only a brilliant WW2 film but a guaranteed tear-jerker, so I watched it with a guarded attitude, not wanting to give in to whatever emotional manipulation was in store. I'm quite glad I did, because while the film does not manipulate you and I was locked out of sharing the intense impact it has on others, another aspect does open up when you approach it in this way.
The firefly metaphor can't help but encourage readings of the film as a critique of the senseless destruction of war. The fireflies represent the people caught up in the war, and more broadly of the brief lives we all lead, and the fate we all share. In one scene, they remind the protagonist of watching a naval parade, and all the proud, patriotic and violent feelings the spectacle stirred within him. He sits up shooting an imaginary machine gun at the air, but the night is peaceful, underlining the embarrassing nature of the outburst.
But there is more buried under this rather unsubtle metaphor. For one, the director has made explicitly clear that the film does not contain a pacifist message. Instead, he draws attention to how the brother and sister fail to survive because of their decision to isolate themselves from kith and kin. This is a difficult perspective to get because we are so invested in Seita and Setsuko's story and their aunt really is mean and conniving. Nonetheless, it is inescapably true that their decision to live apart and alone dooms them both in the end. Independence and individualism is seductive but dangerous. When the film mourns the death of Setsuko, we don't look back to her life before the film starts. We only get images of her playing in the cave – when she was most free, but also when she was most vulnerable.
The end of the film shows the ghosts of Seita and Setsuko looking over a city, an effect Scorsese pinched for his Gangs of New York. Japan's current prosperity is built over the suffering of the generation that experienced the war, and Isao Takanaka may have intended the shot to be a pointed reminder of that fact. If so, the siblings emerge as more noble than the decadent present generation, but they have also made the same mistake – succumbing to a very modern individualist ideal and rejecting the ties that bind a community together.