A prominent journalist has explained why James Mattis' warning needs to be heeded by the NATO alliance.
The new US Secretary of Defence warned NATO members on Wednesday they need to meet the minimum spending of 2% of their gross domestic product.
He said that American taxpayers can't carry "a disproportionate share" to defend Western values, in a comment that echoed Donald Trump's statements on the alliance.
Currently, only five countries in the alliance meet the spending requirements.
Robert Fox, defence editor for the London Evening Standard, claimed that in terms of foreign affairs, Mattis is potentially the single most influential person in the US government.
He told Julia Hartley-Brewer: "Mattis is a very intelligent man - he’s easily the best.
"He’s saying: 'Come on Europe, you need to be more practical.'
"It’s not the tweets from the leader himself, it’s Jim Mattis that counts.
"Mattis is possibly - in foreign affairs - the most powerful figure in the US administration."