So what’s behind Michael Moore’s secret war movie?

A bona fide surprise film is rare in modern moviedom — it’s hard enough to keep a minor casting secret these days, let alone word of an entire project.

So props to Michael Moore, who was able to shoot and edit a complete movie, titled “Where To Invade Next,” without word getting out (until this morning, when it was announced as a world premiere at the Toronto international Film Festival).

This despite the fact that a) Moore apparently has interviewed some pretty big people in the military-industrial complex b) he’s Michael Moore, and thus not exactly unknown and c) this is the age of social media, when someone can’t eat a pancake without a photo winding up on the Web.

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Now that his media secrecy has been lauded, though, comes the bigger question: What in the name of David Petraeus is this film?

Here’s what we know:

“Where To Invade Next” (stylized as “W2IN’ by Moore) is a documentary, his first since “Capitalism: A Love Story” six years ago. It has no distributor on board, and it’s not clear how many distributors have even seen it, though Moore did allude to the film being “in theaters over the holidays later this year.”

“Invade” does center on the U.S. military. A person familiar with the project said it was a movie about the armed forces with surprising themes that are best appreciated if one goes in knowing as little as possible.

Moore offered at least a few hints in a short Periscope chat after the announcement Tuesday morning. He called it a “film of epic nature” and said the thrust was the self-perpetuating nature of American warfare — or as he put it: “This constant need, it seems, to always have an enemy — where’s the next enemy? — so we can keep our whole military-industrial complex alive.”

He also tweeted an image, presumably used in the film or its marketing, that shows a number of very sartorially proper military men sitting around a table. (You can see it above.) Turns out, it’s a picture of the U.S. Joint Chiefs of Staff from 1983, though whether the film actually delves into foreign policy from that era or is just a striking Cold War-era photo is an open question.

Moore’s six-year movie absence has been motivated by many things, but certainly the presence of a Democrat in the White House may not be entirely coincidental.

Toronto documentary section chief Thom Powers, who saw the film several weeks ago and decided to give it a spot in the festival selection, said the timing may have other roots as well.

“You get the sense he’s been saving himself to say something special, to say something meaningful, and that’s what this film is,” Powers told The Times on Tuesday.

In the festival announcement, meanwhile, organizers officially described “Invade” as a piece that “may be his most provocative and hilarious movie yet” in which he “tells the Pentagon to ‘stand down,’ he will do the invading for America from now on.”

Some of those descriptions evoke “Fahrenheit 9/11,” which became a major hit and talking point more than a decade ago. Of course, that movie came out in the early, building stages of the wars in Iraq and Afghanistan and tapped into public anger about military incursions there. Populist foreign-policy ire in this era is as likely to be aimed at perceived dovishness of the Iran deal as at American military overextensions.

Still, Moore had hoped to nudge public sentiment in a presidential election in 2004 when he and the Weinstein Co. opted to release “Fahrenheit 9/11” four months before Americans went to the polls. Voters soon will be making a return. So is Moore.

Twitter: @ZeitchikLAT

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