( CNN) US President Donald Trump has arrived in Brussels with a clear message: NATO members must hike their defense spending and they must do it now.

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 unfair.”

A White House spokesman made clear last week the President will tell NATO that the US is not the world's piggy bank .

Is the 2% threshold reasonable?

While the 2% threshold has eluded some members — whom Trump has slammed as “delinquents” — most have boosted their spending in real-dollar figures in recent years.

NATO was formed after World War II and on the principle of collective defense — an attack against one or more members is considered an attack against all. Each nation sets very little money in a NATO pot of monies. The idea is that each nation should have its own strong defense force, which requires a certain level of spending by individual members.