President Trump said he would impose a 100 percent tariff on movies “produced” outside the United States, proclaiming in a social media post on Sunday that the issue posed a national security threat.

Mr. Trump said he had authorized Jamieson Greer, the United States Trade Representative, to begin the process of taxing “any and all Movies coming into our Country that are produced in Foreign Lands.” Mr. Trump added, “This is a concerted effort by other Nations and, therefore, a National Security threat.”

The Motion Picture Association, which represents the biggest Hollywood studios in Washington, declined to comment. The association’s latest economic impact report, based primarily on government data and released in 2023, showed that the film industry generated a positive U.S. balance of trade for every major market in the world.

As is often the case with Mr. Trump’s declarations on social media, it was not entirely clear what he was talking about. Did he mean any movie, including independent foreign-language films destined for art house cinemas and movies that play exclusively on streaming services?

Would such a tariff apply only to movies receiving tax incentives from foreign countries — or to any movie with scenes shot overseas? What about postproduction visual effects work? A single superhero movie can often involve a half-dozen or more specialized firms scattered around the world.

Technically speaking, the vast majority of movies shown in American cinemas are produced in the United States — scripts written, preproduction planning handled, principal actors cast, footage edited and sound added. But Hollywood has increasingly turned to foreign locales for the cameras-rolling part of the moviemaking process because, as with so much traditional manufacturing, it is much cheaper.