Trump's visit is expected to attract large protests in London and elsewhere in Britain.
In the run-up to the meeting, Trump has been antagonizing American allies while placating Russia's authoritarian President Vladimir Putin. USA doesn't have and won't have a better ally than EU. "We spend on defense much more than Russian Federation and as much as China", Tusk wrote.
Trump also said the European Union is taking advantage of the United States.
Referring to Monday's planned summit with Russian president Vladimir Putin in Helsinki, Mr Trump added: "And I have Putin".
While some nations have taken that request (or demand) seriously and vowed to stick their hands deeper into their pockets, European Council President Donald Tusk cautioned the President: "it is always worth knowing who is your strategic friend and who is your strategic problem". Not fair to the U.S. taxpayer.
He added: "Not fair to the USA taxpayer".
The meeting comes at a time of fraying relations across the European Union, so will anyone stand up to the US President?
In office since January 2017, the USA president has been openly critical of many of NATO's practices, with most of his ire focusing on Washington spending relatively more money than other member states.
Mr. Tusk chided Mr. Trump while the president was en route to Brussels.
Europe's biggest economy is on course to spend just 1.24 per cent of GDP on defence in 2018. In March 2016 he said it was "obsolete and disproportionately too expensive" and on Monday, ahead of the summit, he suggested (on Twitter, of course), that "by some accounts, the usa is paying for 90% of Nato" and on Tuesday raged: "NATO countries must pay MORE, the United States must pay LESS". We believe that the NATO alliance strengthens America because we speak with 29 voices to a Russian Federation that is one, to an Iran that is one, to a North Korea that is one.
Before boarding Marine One to start his European trip, Trump spoke with reporters on the south lawn of the White House, saying, "So I have North Atlantic Treaty Organisation, I have the United Kingdom which is in somewhat turmoil, and I have Putin".
The meeting will be closely watched to see whether Trump will rebuke or embrace Putin, who has repeatedly denied the allegations of election meddling, in spite of evidence to the contrary.
In March, NATO released the figures for 2017 and only a handful of countries hit or bettered the two percent target.
Trump has said maintaining a strong personal relationship with Putin is in America's interest and has signaled to allies that he trusts his own instincts in dealing with Russia's president. "That's a fundamental inconsistency in the pro-NATO position", Mr. Desch said.