Trump has long called on other NATO members to spend at least 2% of their gross domestic product on defense, as the transatlantic alliance's own guidelines recommend.
He has a point. The US spent more than 3.5% of its GDP on defense last year, and far more in previous years, while only three other member nations -- the UK, Greece and Estonia -- passed the 2% threshold last year, NATO data shows.
Poland slipped in 2017 below the benchmark to 1.99%.
"I think these countries have to step it up, not over a 10-year period, but they have to step it up immediately," Trump said in Brussels on Wednesday, on day one of the summit. NATO allies in 2014 agreed to try and reach their 2% target over a decade, by 2024, too long a time frame for Trump. He pointed specifically to Germany as a wealthy nation that could boost spending now.
There was never any doubt Trump would come down hard on his mostly European allies in the 29-member alliance. He has spent recent days on Twitter slamming NATO as "very