VIETNAM-WAR movies come in two categories: films about the war itself, and films about its aftermath, when soldiers returned home and faced readjustment to ordinary life. That second category includes such pictures as ``Coming Home'' and parts of ``The Deer Hunter.'' The latest addition to this group is a small but incisive new film called ``Jacknife,'' about the complicated relationship of two veterans who served in Southeast Asia together. Much of the picture isn't really about Vietnam or the war at all; it's about a friendship, a romance, and the sometimes clashing personalities of the people involved. Vietnam lurks in the background of all the characters' memories, though, giving their relationships an unexpected depth and complexity.
The plot begins when two old vets get together: Robert De Niro as the happy-go-lucky one, dancing through life on the strength of his own high spirits, and Ed Harris as the depressed one, who never quite recovered from the trauma of combat. He drinks a lot, gets little joy out of life, and shows occasional signs of erratic behavior.
He's not alone in seeming mentally unstable at times - even Mr. De Niro's usually cheerful character can fly off the handle and become wildly self-destructive. But he goes to a self-help group for veterans, and he spends time with a counselor, thus keeping his instability under control, if not really cured. The story kicks into high gear when he falls in love with his old buddy's sister, played by Kathy Baker in the movie's third excellent performance. Her brother tries to stop this romance, for reasons that aren't completely clear until we learn about a memory the two veterans share - concerning another buddy who died in combat, and a failure of nerve that's still tormenting one of our heroes after all this time.
There are only a few wartime scenes in ``Jacknife,'' and that's just as well, since these moments are not particularly striking. (The picture's modest budget has something to do with this.) But the love-story aspect and the relationship of the two veterans are handled with commendable sensitivity. De Niro is always convincing, yet always unpredictable in the seemingly cheerful role; Mr. Harris becomes steadily more interesting as the story develops; and Ms. Baker is perfect as his sister - attractive enough to make a believable girlfriend for De Niro, yet shy and plain enough to explain why she's never found anyone before now.
The movie was directed by David Jones, whose other films - ``Betrayed,'' from a Harold Pinter screenplay, and ``84 Charing Cross Road,'' about a long-distance love affair with books - have also been literate, human-scaled dramas. He's the kind of director American films need badly right now: a moviemaker with no interest in action or special effects, but a gift for making the deepest emotions spring to life before our eyes. ``Jacknife'' is a small movie, but it won't be forgotten in a hurry.