What was the symbolism of the field where Kiowa died?
The field where Kiowa died also symbolizes the personal tragedy of millions of young American soldiers who gave their lives for a lost cause. The Vietnam War was lost due to political factors beyond the average soldier's control.
The swampy field symbolizes not just a loss of innocence (many American soldiers who fought were only 18-24 years old) but also a loss of hope. Many soldiers, like Kiowa, died serving their country in a foreign land. As an added tragedy, their families never recovered their bodies. Of the soldiers who died or were classified as missing-in-action in the Vietnam War, a few thousand remain unaccounted for today.
In the story, O'Brien relates that Kiowa's burial in the smelly, swampy field makes him one with the war. After the war, O'Brien receives a letter from Bowker, who admits that the war has destroyed his courage:
"The thing is," he wrote, "there's no place to go. Not just in this lousy little town. In general. My life, I mean. It's almost like I got killed over in Nam...Hard to describe. That night when Kiowa got wasted, I sort of sank down into the sewage with him . . . Feels like I'm still in deep shit."
Bowker relates in no uncertain terms his opinion of the war. To Bowker, the "stench" of the war (as symbolized by the swampy field) will never leave him. It follows him everywhere he goes. Essentially, the war resulted in two tragedies: the loss of young lives and (for surviving soldiers) the loss of hope and purpose. The memories of the dead and the travesties of war continue to haunt the living. O'Brien relates that Bowker hanged himself in a YMCA locker room in 1978.
So, the field symbolizes (among other things) the tragedy of lost innocence, hope, and young lives.
Water symbolism runs throughout The Things They Carried. Water usually symbolizes baptism and renewal, but in O'Brien's novel it infects and kills. But, years later, the Song Tra Bong, the river whose banks overflow and drowns Kiowa, is a place which triggers O'Brien's memories and inspires his stories.
Mary Anne survives swimming in the Song Tra Bong, whereas the innocent soldier, Kiowa, is swallowed by it. Morty Phillips swallows a mouthful of it and dies, and Bowker, the professional soldier, kills himself because of its stench. The Song Tra Bong is both a rite of passage and a sirens' song, and once baptized by it, one longs to return to it (to bury Kiowa's moccasins) and be tortured by it (Bowker's suicide). And lest we not forget that O'Brien could have been saved from the whole Vietnam experience by a swim across a different river.
The shitfield where Kiowa died is a metaphor for the Vietnam War itself. The land war in Asia is often referred to in similar terms as a "quagmire," which is literally a swamp, bog, marsh, or mire. Figuratively, a "quagmire" is a mix-up, mess, predicament, quandary, confusion, sticky situation, or dilemma. Personally, I like "shitfield" better than them all. The Vietnam War stinks and kills, literally and figuratively.
So, the field symbolizes Vietnam, the quagmire of the Vietnam War, the death of Kiowa, the death of Tim's innocence, and the place where so many of Tim's memories are buried that he must return to it, with his daughter, so she too can tap into its source for storytelling.