It's not just that Tropic Thunder is the funniest American comedy of the year by a mile. It's that Tropic Thunder is one of the nastiest American comedies in what feels like decades, taking a well-trod subject - how movies are made - and attacking with a savage gusto that calls to mind Sunset Blvd. and The Player and not much else. From the evidence of the film, you might well conclude that director/co-writer/star Ben Stiller hates making movies more than anything else on earth.

The likelier case is that Stiller - a man whose blazing comic sensibilities have been stagnating under an ever-expanding pile of Hearbreak Kids and Along Came Pollys, among countless others - understands that if satire is to work, it can't be the thin, milquetoast kind of satire that mocks but only with affection. Satire must take no prisoners, it must leer with haughty superiority at the pathetic feebleness of its victims. And Tropic Thunder is a very good satire - I honestly cannot remember the last significant American movie prior to now that lacked even one sympathetic character, as a point of entry for the audience if nothing else. Stiller and co-writers Justin Theroux (an actor in his writing debut) and Etan Cohen (a former Mike Judge crony) never even feint at giving us such an identification point: for almost two hours, we're tossed into the company of complete assholes, and it's funny as hell.

Opening with a serial gag that I'd just as soon not spoil, save to say both that I am terrifically impressed by the filmmakers' parsimonious introduction of the characters, and that the funniest single gag I've seen in a movie theater in 2008 is involved, the film quickly slides into its story: there's a war movie on, starring fading action hero Tugg Speedman (Stiller), juvenile comedian and fartsmith Jeff Portnoy (Jack Black) and multi-Oscared Method actor Kirk Lazarus (Robert Downey, Jr.). Directed by neophyte Damien Cockburn (Steve Coogan) from the Vietnam memoirs of "Four Leaf" Tayback (Nick Nolte), "Tropic Thunder" is headed for absolute failure, going weeks and millions of dollars overbudget in its first five days (shades of Apocalypse Now). The crazed survivalist Tayback suggests to the hapless director a scheme: take the actors into the jungle, leave them there, and film their pants-shitting terror as pyro expert Cody (Danny McBride in his second consecutive Wednesday-opening action-comedy) blows the fuck out of the jungle around them. (The notion of actors going nuts in the jungle because what they go through is too close to the movie they're making, also contains shades of Apocalypse Now). Things get a little difficult when the actors stumble into the territory controlled by a heroin-ring, and find themselves assaulted with proper bullets and explosives.

You can't believe for a minute that it's actually possible to shoot a movie like that, which is exactly the point: the chief message of Tropic Thunder is that people who make movies, especially actors, are tremendously blinded by their own self-importance. Forget if something is remotely plausible or sensible, if it strokes the artist's ego, it justifies itself. Nowhere is that clearer than in the already (in)famous detail that Downey Jr's Lazarus, a white Australian, is playing an African-American in what basically amounts to blackface. A parody of the body-modifying tricks made famous by performers like Robert De Niro and Christian Bale, the joke is that Lazarus thinks that merely by darkening his skin and aping the vocal styles of black sitcom stars from the 1970s, he's done all he needs to do to inhabit a role that he is fundamentally unsuited for - an absurdity pointed out, repeatedly, by the project only legitimate non-white star, rapper/actor Alpa Chino (Brandon T. Jackson).

The now-dormant cries of "racism" miss the point, just as much as the recent contretemps over the line "Never go full retard", a line that certain advocacy groups want to decry as biased against the mentally handicapped. Not since Scorsese made a certain deeply pious film that was accused of rabid anti-Christianism has a controversy so utterly missed the point: the context for that line is a conversation between Lazarus and Speedman about the latter's film "Simple Jack", in which the Method actor casually and brilliantly describes the sort of idiotic thinking behind such genuinely problematic films as Rain Man, Forrest Gump and I Am Sam. The actors never seem to even realise that mental retardation exists in the world; for them, it's a chance to showboat and aim for an Oscar nomination. I don't like to say mean things about other people, but I can only politely describe the smart and less-smart people who are actually offended by this gag as terribly naïve; it must take a deeply thoughtless viewer to conclude that we're laughing at "Simple Jack", rather than the morally-defective actor who thinks that "Simple Jack" is actually a sensitive movie.

Making fun of actors is definitely swinging at low-hanging fruit, but ordinarily these kinds of pictures have some measure of affection behind them; I detect no affection in Tropic Thunder, only viciousness. And viciousness, as any honest comedy lover will tell you, is at the heart of everything that's funny. I must confess to my awe at just how funny the film actually is, at that: Stiller is an uneven performer, even here, but he proves to be a tremendously gifted director, pacing gags at a nearly perfect pitch (the film is funny throughout, even when it looks like people are about to die, but there are plenty of moments when he doesn't keep trying to hammer home tiny gags, letting us catch our breath from all the big ones), and relying on well-timed reaction shots to a degree that one hardly ever sees these days. Unsurprisingly, since there's nothing that's trickier to get right than a reaction shot. Stiller gets it right, frequently. He also wrings a tremendous amount of simple parody - satire's nicer cousin - from the conventions of the war genre itself, visually quoting just about every modern war movie you could name, and bringing along the tremendously gifted John Toll (who shot that particular masterpiece of the modern war genre, The Thin Red Line) to make sure everything looks as lush and green as all the Apocalypse Nows and Platoons of the world.

Is it perfect? No. For starters, while Downey is in rare form, Stiller is not, and Black hardly gets to do anything in a disappointingly flat role. And like seemingly every American comedy of the last five years, it's too long by some ten or fifteen minutes in the middle. Not to mention the unpressing need for yet another Hollywood satire.

But there's so much greatness going on, it seems positively mirthless to harp on those things. This is a tremendously funny and smart comedy, layered all the way into the real world (there are multiple gags that only work if we know who Robert Downey, Jr. is in life), and a tremendous performance by a certain actor in a role that has been spoiled all over the web, but that I'd rather leave a surprise - it's the one that's a parody of Harvey Weinstein, just so you know. Smart, funny movies hardly ever come around these days, and we've had two in under ten days - let us not try to demand that Tropic Thunder be flawless, and instead praise it to the skies for being close enough that perfection can be glimpsed off in the distance.