While closer to Our Man in Havana then Brighton Rock in subject matter, the underlying theme is closer to the later, being another take on the evils of strong morals versus common-sense somewhat-amoral rightness. Bracketing the two too closely together would be somewhat simplifying, though.
In comparison to this novel, the characters of Brighton Rock are caricatures. Here, everyone's a shade of grey. Even the people made out to be black or white are revealed to be just pigeonholed versions of the real characters, adjusted by the protagonist's viewpoint.
The main character, Fowler, is a journalist reporting on the war in Vietnam, who tries to pride himself on his detachment. Into his life comes Pyle, the eponymous quiet American, who attempts to kick start an ideological Third Force against Communism, and steals Fowler's girlfriend into the bargain.
The device of making the main character a journalist, and setting the novel in a war zone is very effective for both exploring detachment and making sure that abstract concepts are tested against real suffering. Greene's journalistic background does show through, and very positively. It works well as both a study of individual morals and wider issues of colonialism and imperialism.
The nearest thing this reminds me of is Le Carré, presumably because he learnt from Greene. But this is not exactly a spy novel, nor a war novel, but well, a Greene novel.