How did Homer's dad die?

The circumstances concerning the death of Homer's father in The Human Comedy are never fully explained. As the novel is set during World War II and one of the other brothers is in the army, we could perhaps assume that he too has died as a result of the war.
Ultimately, however, the way Homer's father died isn't really relevant to the story. What is relevant is the fact that he is dead, because this majorly impacts all of the other characters.
Homer, our main character, is only fourteen and should be able to live his teenage life. However, due to the fact that his father is dead and his older brother is in the army, he must step up to the situation and quickly become an adult. He acts as a father to his younger brother, Ulysses, and is also the main provider for the whole family. He delivers telegraphs for a living and goes to school at the same time. 
Homer's dad also serves as a ploy to strengthen the presence of the theme of death within the novel. The deceased father is often talked about; Homer's mother even imagines talking to her dead husband about family problems. The idea is that the dead live on in those who remain alive.
Throughout the novel, we encounter the theme of death at various times. Homer, in particular, must deal with it often, as his job as telegraph messenger involves bringing bad news to the families of those killed during the war. Although Homer does not know the families, the task is hard to perform, and he comes to dread it. Homer then encounters death personally two more times. The first death is that of Mr. Grogan, who works in the telegraph office and dies of a heart attack while typing a telegraph. The second death is that of Marcus, Homer's brother. Homer then brings home Mr. Grogan's last telegraph, which was in fact addressed to Homer's mother to inform her of her oldest son's death.