A U.S. withdrawal would not only cost Ukraine about half its arms, it would also deprive the country of many of its “most effective” ones, according to Ben Barry, senior fellow for land warfare at IISS.

Some U.S. systems such as Abrams tanks can be switched for European ones, like Leopards. Meanwhile, the F-16s are all supplied by European countries, not the U.S., while there are competitors for Patriot air defense system like the SAMP/T, Germany’s IRIS-T and Norway’s NASAMS.

But Lithuanian Foreign Minister Kęstutis Budrys told POLITICO that there is “certain weaponry, certain equipment [that] Europe cannot substitute for the U.S.,” citing air defense systems and some long-range munitions.

“It will take us time and we’re working on it … it is a challenge because there are no direct substitutes, or quick shortcuts, but we can step in with finances to help Ukraine produce more itself,” he said.

While Europe has increased its defense spending in recent years, and has made efforts to increase procurement and domestic production of ammunition, losing the U.S. will create a gap.

“The Europeans have maximum six months to step up,” said Ukrainian lawmaker Ivanna Klympush, adding that they “just don’t have what the Americans have.”