In 2022, as fears mounted over Europe’s missile defense capabilities against Russian strikes, the Biden administration granted permission for Germany to build a factory to manufacture state-of-the-art Patriot air defense interceptors.

That factory has yet to produce a single Patriot — a sign of the time it can take and how difficult it can be to ramp up production of American weapons abroad, even after winning approval from the United States government to do so.

It is a cautionary tale for Ukraine, whose leaders expressed confidence that they could quickly start Patriot production after President Trump said on Wednesday that he would grant Kyiv a long-sought license to produce the interceptors.

Only two other American allies — Germany and Japan — have similar licenses, and neither scaled up nearly as quickly as Ukraine hopes to.

And European leaders still often rely on American-made weapons, even as they try to build their own alternatives. Friedrich Merz, the German chancellor, acknowledged as much on Thursday, announcing that his government had reached a deal with the United States to buy American-made Tomahawk missiles.

“A manufacturing license is not a quick fix,” said Satoru Mori, a professor of contemporary international politics at Keio University in Tokyo, referring to Japan’s experience developing Patriots. “It is an incredibly complex aerospace engineering feat.”