Secretary of Defense James Mattis appealed to his British counterpart, Gavin Williamson, by threatening that France could overtake the United Kingdom as the U.S.’s top military ally in Europe if the U.K. doesn’t up its contribution to defense spending.

U.S. is increasing pressure on the U.K. ahead of President Donald Trump’s trip to Brussels later in July for a NATO summit where he will try to goad European countries into reaching goals of at least 2 percent economic output to defense every year by 2024. (RELATED: Mattis Heads To Brussels For NATO Meeting As EU Seethes About Trump Tariffs)

Mattis challenged Williamson to convince Prime Minister Theresa May and Chancellor Philip Hammond to rise to the occasion and exceed the 2 percent goal, which they have already been meeting, according to a leaked letter between the two defense secretaries.

The two U.K. leaders have ruled out contributing anything more to their defense budget, despite France’s President Emmanuel Macron pledging the opposite. France is below the NATO goal, coming in at 1.7 percent, but has committed to funneling billions of dollars into its budget from now until 2025.

“The UK maintains the biggest defense budget in Europe and we have been clear we will continue to exceed NATO’s 2 percent spending target.” a Ministry of Defense spokesman told the Guardian Monday.

Still, the reluctance to move beyond the target goal has positioned France to take the U.K.’s spot as a key European defense ally to the U.S., a position it has been vying for despite a former contentious relationship between France and the U.S.

“It is not for me to tell you how to prioritize your domestic spending priorities, but I hope the UK will soon be able to share with us a clear and fully funded forward defense blueprint that will allow me to plan our own future engagement with you from a position of strength and confidence,” Mattis said in the letter sent to Williamson on June 12.

The U.S. hopes for an update on how the U.K. plans to proceed on defense spending by the time the Brussels summit rolls around on July 11.

“A global nation like the UK, with interests and commitments around the world, will require a level of defense spending beyond what we would expect from allies with only regional interests. Absent a vibrant military arm, world peace and stability would be at further risk,” Mattis warned.

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