Grave of the Fireflies is a very old animated movie set during World War II about a boy and her sister struggling to survive. Despite its obscurity at the time of its release, it has since gained more well-deserved recognition from a wider audience.
Because of its history, Grave of the Fireflies already has a lot of written reviews and summaries, so I won't bother with either; you can read Roger Ebert's here. As I understand it, Roger Ebert is a renowned film critic; however, I strongly believe that his interpretation of this movie is incorrect. Perhaps he was deluded by that strange feeling that is invoked by many Japanese works, perhaps his search for that perfect movie left him desperate, I can only guess. Ebert calls Grave of the Fireflies a war film. It is not a war film; rather, it is a film about the tragedy that befell two siblings who, forced to grow up too quickly, isolated themselves from society.
His mistake is understandable because the first twenty minutes or so Grave of the Fireflies certainly portrays itself as a war film might: we see the devastation caused by the Japan firebombings, leaving the protagonist homeless and motherless. We see people running in panic and dead bodies everywhere. We see Seito and Setsuko's town burnt to the ground. However, this façade is shattered by one specific event: when Seito asks for more food, his aunt refuses, criticizing Seito for not contributing to society. Due to his immaturity, he doesn't work, and due to his pride, he refuses to apologize to his aunt, who might still have taken pity on him and his sister. Seito moves out and leads a solitary life with his sister, depending only on thievery and resourcefulness, and the tragedy that follows can only be attributed to Seito's failure, not the war. Had the film shown Seito trying unsuccessfully to find a job, the film might have taken on a different bent, but it did not because that was never the intention of its makers. A war film would highlight the suffering caused by war, but from this point on, Grave of the Fireflies is a classic tragedy, highlighting how Seito's immaturity leads to his and his sister's deaths.
However, director Isao Takahata repeatedly denied that the film was an anti-war anime. In his own words, "[the film] is not at all an anti-war anime and contains absolutely no such message". Instead, Takahata had intended to convey an image of the brother and sister living a failed life due to isolation from the society and invoke sympathy particularly in people in their teens and twenties.
Before digging into any work of art, it's always a good idea to look into its history as well. This applies mainly to literature, but also to film, music, and video games. Grave of the Fireflies is an adaptation of an autobiographical novel by the same name. Akiyuki Nosaka, the author of that novel, intended for it to be an apology to her sister, who died of malnutrition during wartime 1945. Neither the book nor the film was ever intended to be about the war, merely about a tragic, yet almost unnoticeable death occurring in the war's midst. In the titular scene where Setsuka makes a grave for the fireflies Seito caught, war has no place. There's only two siblings, isolated from society, mourning the fragility of life.