I will confess upfront that I have never seen a Rambo movie before — yes, there are serious gaps in my film education. I’m sorry. But I was with this new incarnation for a goodly while, partly because it’s lean and spare and Sylvester Stallone (Rocky Balboa) is so damn surprisingly good at being an iceberg: 10 percent rock-solid implacability on the surface and 90 percent hidden and unexpected depths. That’s exactly where his John Rambo, Vietnam vet now lying low in Thailand, is, until a group of American missionaries comes through looking for a ride upriver into wartorn Burma. Stallone — who wrote and directs — gives us a merciless and graphic look at the brutality of the civil war there (it’s more like genocide), but then he doesn’t know where to go from there. Spare is good: so spare that essentials are missing is not. Rambo’s detachment and cynicism is challenged by the missionaries’ dedication to a seemingly hopeless cause, but much of the subtext Stallone seems to believe is present isn’t, and so his transformation from a man who doesn’t care into one who does is perplexing in its context. The only place to go from there is into cruel and pointless action-movie violence, when — inevitably — the missionaries require rescue from the vicious Burmese military junta. I say “pointless” because the film fails even in its apparent goal of underlining the idea that only brutal men can fight brutal men. That’d be a conclusion I’d disagree with, if the movie were able to cohere around it, but all it seems capable of managing is bodily mutilation in the name of supposed morality.