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.”

How Japan and Germany Won Patriot Licenses

In the early 2000s, Japan’s leaders grew concerned that their homegrown defense systems were not good enough to combat North Korea’s nuclear and missile programs. The United States granted it a license in 2005 to produce more advanced systems, including Patriot missiles known as PAC-3 “hit-to-kill” interceptors, in partnership with the manufacturers Raytheon and Lockheed Martin.

The American decision was a “natural progression after many years of defense cooperation and trust,” said Jeffrey W. Hornung, an expert on Japan at RAND, a research group in Washington.

It took three years for Japan’s military, called the Self-Defense Forces, to conduct the first successful test of PAC-3 systems. It now produces as many as 30 Patriots a year by some estimates.

Germany’s license was awarded amid fears that Russia, having focused for years on Ukraine, might one day turn its missiles on Western Europe. The license was followed by a 2024 contract for a German factory to supply up to 1,000 missiles to Spain, Germany, the Netherlands and Romania. The new factory, in the city of Schrobenhausen in southern Germany, is expected to begin producing missiles next year.

Why Ukraine Can’t Just Buy Patriots From Germany

Once Germany’s Patriot system is operational next year, it will initially be able to deliver missiles faster to Ukraine in the short term than Ukraine will be able to build under its own production license. Germany and its allies must supply Patriots to Ukraine “as quickly as possible until Ukraine is able to produce them on its own,” said Anton Hofreiter, a German lawmaker and weapons expert.

Yet Ukraine cannot completely rely on allies for support, because the war in Iran, as well as the war in Ukraine, have reduced the global stockpile of Patriots and fueled a demand for more missiles that exceeds production capacity. In 2023, Japan started selling some of its home-produced Patriots back to Washington, in part to backfill American shipments to Ukraine.

The ability for Kyiv to build its own factory will ultimately increase its supply of missiles — and make it less dependent on its allies for defense.

Why Ramping Up Is Hard

To make a Patriot missile, a country needs to find and license the roughly two dozen suppliers involved in various parts of the production process.

Germany’s efforts, however slow, have still been accelerated by existing engineering expertise. Even before Berlin got the license to make its own Patriots, the operators of its new Patriots production site had for years held a contract to maintain American-made Patriot systems, which included relationships with suppliers and toolmakers.

Ukraine, in contrast, will be starting from scratch — at a difficult moment.

Japan had the luxury of stockpiling Patriot missiles during a time of peace in the region. Ukraine does not.

“The plane has to be built while it’s flying,” Mr. Hornung, the Japan expert, said. “They’re going to want to hit the ground running on this as quickly as possible, but you just have to make sure the correct safeguards are in place.”

Kiuko Notoya contributed reporting from Tokyo.