The Trump administration has included Britain, Washington's most loyal battlefield ally in the wars in Iraq and Afghanistan, among countries being sent letters rebuking them for spending too little on their militaries.
Secretary of Defense Jim Mattis threatened to replace Britain with France as its main military ally unless London ramps up its defence spending, in a letter sent on June 12 and reviewed by Reuters on Tuesday.
The strongly-worded letter from Mattis to Britain's Defence Secretary Gavin Williamson noted that Britain was one of the few NATO allies that already meets the alliance's target of spending 2 per cent of economic output on the military.
But it said that was not good enough. Britain's global role "will require a level of defence spending beyond what we would expect from allies with only regional interests", Mattis wrote.
"I am concerned that your ability to continue to provide this critical military foundation ... is at risk of erosion," he wrote. He asked for a "clear and fully funded, forward defence blueprint" from Britain, and said he hoped for an update at a NATO summit next week.
The letter shows US President Donald Trump's determination to pressure NATO allies to significantly increase military expenditure, ahead of the summit in Brussels.
Since Prime Minister Tony Blair vowed to stand "shoulder to shoulder" with the US after the September 11, 2001 terror attacks, Britain served as the main US ally in both Iraq and Afghanistan, losing more than 600 service members. The Afghanistan war was commanded by a British general during one of its most violent phases while US forces were tied up in Iraq.
"It is in the best interest of both our nations for the UK to remain the US partner of choice," Mattis wrote, noting that France was increasing its spending. "As global actors, France and the US have concluded that now is the time to significantly increase our investment in defence."
Reuters has seen letters from the Trump administration to Belgium and Norway, written ahead of next week's NATO summit, demanding those countries increase defence spending in line with the Western alliance's 2 per cent target. The New York Times has reported that Trump also wrote such letters to Germany and Canada.