Gen Mattis will meet his Nato counterparts this week in Brussels, their first meeting since Mr Trump took office after a campaign in which he repeatedly questioned the organisation and its reliance on Washington. The president has also questioned US support for European integration.
Nato allies want Mr Mattis to flesh out Mr Trump’s demands for a steep rise in defence spending and a bigger role for the organisation in counter-terrorism operations. “We are lacking clarity regarding immediate issues which are on the table,” said a senior Nato diplomat.
“So far we have just general statements and general demands, but what does it mean in concrete terms?”
Mr Trump rattled allies when he branded Nato “obsolete” after taking office. But soon after he backed the alliance in talks with Theresa May, UK prime minister, and later with Jens Stoltenberg, the top civilian in Nato.