Why was Kiowa's death used as a metaphor for the Vietnam War?

The character of Kiowa in The Things They Carried is significant for a number of reasons. The narrator's best friend, he is of Native American heritage and shows genuine concern and kindness to his fellow comrades. He is decent, courageous, and admired by the men he serves with. By contrast, Kiowa is also remembered in the novel for his horrific death by drowning in a field of sewage, which serves as a metaphor for the Vietnam War as a whole.
In the story "Speaking of Courage," Norman Bowker gives a detailed (and nauseating) description of the field where Kiowa meets his death. He describes it as "a deep, oozy soup" that carries a "dead fish smell" caused by the field's function as a toilet for a Vietnamese village. We later learn that Kiowa dies when the company takes fire and the field is so disturbed that it "explodes." When Norman unearths his body, "there were bubbles where Kiowa's head should have been" and his skin is coated in sewage.
To understand the symbolism of Kiowa's death, we also need to understand the historical significance of the Vietnam War. What started as a provision of funds to the French from the United States during the nation's conflict with Vietnam gradually grew into a prolonged debacle that the United States was unable to win. In the end, 58,220 soldiers died during the war. Many historians have described the Vietnam War as a quagmire, a word that means both a muddy area of land and a challenging situation that is hard to get out of. Therefore, the sewage field can be seen as a metaphor for the messy, dangerous situation of the war itself. "If it had been possible, which it wasn't," Norman thinks, "he would have explained how his friend Kiowa slipped away that night beneath the dark swampy field. He was folded in with the war; he was part of the waste."
http://www.pbs.org/battlefieldvietnam/history/

https://genius.com/Tim-obrien-speaking-of-courage-chapter-15-annotated

https://www.archives.gov/research/military/vietnam-war/casualty-statistics