Agreed. This is one of the most depressing movies ever. Very well made. The pacing is perfect, animation is good in a subtle way. You've got to see it before hearing too much about it. 
spoiler:
I think the most depressing thing about this movie is that the elder brother tried his absolute best to keep his baby brother alive, but his baby brother died in front of him anyways. He was only like 15 or so (correct me if I'm wrong, I watched it like back when it came out in Japanese), and there's so much love between him and his brother shown through their conversation and his attempts to find food for his little brother that you would literally break down when the little brother died. 
This movie really made me hate war (not that I like it before the movie, but it definitely brings up my level of hatred). Both of them would have been living such a happy life if there were no war and their mother did not die.