"Dear Mr President, please remember about this tomorrow when we meet at the North Atlantic Treaty Organisation summit, but above all when you meet [Russian] President [Vladimir] Putin in Helsinki".
Member countries are anxious that Trump will spurn them and threaten to unravel the organization, their fears stemming from the G7 meeting last month. By those metrics, some of the deadbeat members look much more involved.
European Council President Donald Tusk is suggesting US President Donald Trump remember who his allies are as Trump prepares to meet Russian President Vladimir Putin. Separately, doubts about Trump's commitment to European security have pushed EU leaders to boost defense cooperation.
"Dear President Trump: America does not have, and will not have a better ally than Europe", Tusk added.
Mr Trump told reporters before leaving Washington there were "a lot of things going on" in the United Kingdom at the moment.
During a rally last week in Montana, Trump vowed that he would "tell NATO: 'You've got to start paying your bills."' The president also bemoaned that Americans were "the schmucks that are paying for the whole thing".
Trump will focus on Russia's 'malign' activity at summits: U.S
He is due to arrive in Helsinki the evening of July 15 for meetings the next morning with Finnish President Sauli Niinisto. The summit also will include a bilateral meeting and a working lunch between Mr.
Trump also noted that 2% of GDP should be the "bare minimum for confronting today's very real and very vicious threats".
On Nato, President Trump's main objection is that a number of member states have not increased their defence budgets to the target of 2 percent of economic output.
"Is the President just going to focus on defense spending and basically hijack the entire conversation?" says Heather Conley, director of the Europe Program at the Center for Strategic and International Studies. The U.S.is spending many times more than any other country in order to protect them. "I've said that many times for many years".
But NATO chief Jens Stoltenberg pointed out that military spending had been rising in Europe since 2014 and members were making efforts to meet the objective, particularly Germany, which is often singled out by Trump for criticism.
European leaders seem to realize they have a weak argument when Trump blasts them for subpar defense spending.
The president touted that the USA lost $151 billion a year ago on trade and slapped tariffs on other countries in an effort towards free and fair trade. His good behavior-though hopes in this quarter died long ago-would also help reassure allies before Trump meets with Putin that he will not fall for the Russian leader's blandishments or agree to any grand bargains, perhaps in the Middle East, that would be hostile to core Western interests. While last June, NATO expected a 4.3 percent spending growth in 2017, the figure was revised in March to nearly 4.9 percent and further to over 5.2 percent in July. But Trump's claim that the US pays 90 per cent of NATO's total defense spending isn't corroborated by available data. Other NATO members are already pushing back, arguing that their commitment to defense is rising and they back U.S. priorities in other important ways.