Oh, my God, what a sad movie! Grave of the Fireflies is a 1988 animated Japanese film directed by Isao Takahata. Unrelentingly depressing. It's one of the most depressing films I've ever seen, and I've seen Bicycle Thieves.
DVD Talk calls it "one of the most moving and overwhelming war films ever made." Roger Ebert has it on his list of great movies, calls it "an emotional experience so powerful that it forces a rethinking of animation" and says, "it belongs on any list of the greatest war films ever made." Rotten Tomatoes gives it a score of 96%.