Last month, President Donald Trump sent  letters to several NATO allies, including Germany, Norway, Canada and Belgium, criticizing them for not spending the required 2% of their GDP on defense. Trump also is warning that the U.S. is growing frustrated with these countries for not meeting their part of the agreement, according to The New York Times.

The letters fit a pattern of Trump critiques of NATO, particularly of defense spending by other members. NATO members committed to spend 2% of their gross domestic product on national defense at a 2014 summit in Wales, and Trump has repeatedly bemoaned that NATO allies have not fulfilled this commitment.

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“Members of the alliance must finally contribute their fair share and meet their financial obligations,” Trump said during a May 2017 NATO meeting. “Twenty-three of the 28 member nations are still not paying what they should be paying and what they are supposed to be paying for their defense. This is not fair to the people and taxpayers of the United States.”

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In the letter to German Chancellor Angela Merkel, Trump said, “As we discussed during your visit in April, there is growing frustration in the United States that some allies have not stepped up as promised. Continued German underspending on defense undermines the security of the alliance and provides validation for other allies that also do not plan to meet their military spending commitments, because others see you as a role model … It will, however, become increasingly difficult to justify to American citizens why some countries do not share NATO’s collective security burden while American soldiers continue to sacrifice their lives overseas or come home gravely wounded.”

Trump, who used similar language in letters to other leaders, also suggested that the U.S. “might adjust its military presence around the world if its allies do not step and spend more for their own security.”