SAN FRANCISCO — Despite a trade war between the United States and China and past admonishments from President Trump “to start building their damn computers and things in this country,” Apple is unlikely to bring its manufacturing closer to home.

A tiny screw illustrates why.

In 2012, Apple’s chief executive, Timothy D. Cook, went on prime-time television to announce that Apple would make a Mac computer in the United States. It would be the first Apple product in years to be manufactured by American workers, and the top-of-the-line Mac Pro would come with an unusual inscription: “Assembled in USA.”

But when Apple began making the $3,000 computer in Austin, Tex., it struggled to find enough screws, according to three people who worked on the project and spoke on the condition of anonymity because of confidentiality agreements.

In China, Apple relied on factories that can produce vast quantities of custom screws on short notice. In Texas, where they say everything is bigger, it turned out the screw suppliers were not.