The horror of war
Undeniably, the core theme of the novel is the horror of warfare (and the Vietnam War in particular). Like The Things They Carried, The Sorrow of War seems to be a compendium of shattered vignettes. In Ninh's novel, the reader sees war as a compilation of traumatic remembrances, mixed together as one entity. It's in an attempt to share the experience with the reader that Kien subjects himself once more to the horrors of his memory.
PTSD
Kien is purposely triggering episodes of Post-Traumatic Stress Disorder when he collects his experiences for the purpose of writing them into a book. This prolonged experience of PTSD is difficult to bear, but the suggestion seems to be that Kien views telling the truth about what happened to him as part of his duty and that his reward for processing these experiences through language is that he eventually enjoys somewhat peaceful memories of the past and better times.
Language and Writing
The only way Kien can expiate the "sins" of his conscience is by translating that trauma into language. It isn't as simple as trying to capture the experience in language, but it does entail venting one's remembrances despite what logic or authorial structure might initially demand. It is a succumbing, an abandonment; it is letting trauma work itself out on the page. Kien's messy, non-linear novel is not meant to be published, as we hear and observe many times; rather, it is meant to be a cathartic experience. Though language is notoriously unable to fully express trauma, it is paradoxically the means by which many people deal with that trauma.
Failings of Marxist Ideology
The Sorrow of War is open about how hollow the politics of Vietnam feel to veterans and civilians. Indoctrination is a waste of time; the authorities don't seem to have the people's interests in mind. There are no great changes or higher standards of living. People who don't conform to ideological expectations find themselves marginalized or even exterminated. The postwar period bears a facade of glory, but behind that facade is the immense struggle of people trying to put their lives back together. Sloughing off the imperialists is a victory indeed, but few systemic and positive changes result.
Sacrifice
Kien marvels at how sacrifice seems to be an everyday part of the war to the extent that Hoa's remarkable sacrifice is completely unremarkable to those who benefit from it. Indeed, at any moment in the war, a person might be called to give up their own life, limb, or sanity for another. This is both a wonderful thing and a terrible thing, Kien implies, because while it shows that the Vietnamese are noble people, it also shows that war is brutal, unsparing, interested only in churning out more dead.
Memory and the Past
The novel is concerned with traumatic memory: what do we do with it? How does it affect us? Can it serve us, or must we serve it? Kien spends much of the novel being controlled by his memories. He laments the prewar past and anguishes over the immediate past of the war; memories come upon him unbidden. He cannot figure out how to navigate this world until he starts writing. As the memories spill out on the page, often in no particular order or hierarchy, Kien figures out that it is okay to embrace the pure, heroic past before the war and to set aside (mostly, but not completely) the horrors of the war.
The Inability to Truly Go Home
Kien dreams of Hanoi, Phuong, and being home, but when he returns, he learns what scores of soldiers throughout history have learned: it is impossible to go home. Either the place has changed or the soldier has, and for Kien, both have. Hanoi is not what he expected, nor is Phuong, and he knows he has irrevocably changed as well. The authorities and civilians care little for the veterans. Kien has no family left, no Phuong, and no motivation for carrying on. There is only the pressure of memory, and Kien cannot relieve this until he makes his novel into his "home."
