That was the short deadened sound I made when I read the last line of this book last night. Not 'wow' or 'stunning' or 'masterpiece,' just a small involuntary expression of respect for the forces at work in this book that never fully materialize. Yes, it is about the soul -- and in the last hundred pages that word begins appearing with a striking frequency -- and yes, it is about the machine of war -- again, mentioned explicitly a number of times near the end -- but unfortunately it is not a statement or full exploration of either.
The book moves fluidly, and at times gracefully, building characters with depth and sorrow they themselves are unaware of. But at a certain point (1970 for those who've read it) Johnson loses his way. It's as if he has no idea how to draw any of these beings in, so with a number of them he just let's them go, off into the ether never to be read of again. It is profoundly unsatisfying. He then opts out of any responsibility for the fates of these characters by building a 12 year ellipse into the end of the book that leaves the reader with a a chance at speculation but no real knowledge of what has come of them.
All the while, as you've been reaching this point, there are fragments of a larger thematic arc but they remain fragmented. The initial set piece revolving around a man killing a monkey gave me as much insight into Johnson's perspective on war, the soul, and the machine of war as the next six hundred pages.
As an aside, for a heartbreaking book on the ineffable effects of war on the soul, check out Philip Caputo's Vietnam memoir, A Rumor of War