"Very Unfair!" Trump wrote in a second Tweet.
German Chancellor Angela Merkel and British Prime Minister Theresa May will join Trump for the two-day meeting, which comes during divisions among America and its allies over policies on trade and pulling out of the Iran nuclear deal.
Here are some of the key talking points around this year's summit.
Trump has ramped up his rhetoric ahead of the talks - including in three separate tirades on Twitter on Monday and Tuesday - making the summit one of the most hard in years for the military alliance that has underpinned European security since World War II.
But unlike his recent tweets in which he berated allies, Trump struck a more affable tone in person before taking his flight: "We will work it out and all countries will be happy".
Trump may seek to take credit for the increases in European spending that are now registering in government accounts.More news: NATO Braces for Trump Visit, But Fears May Be Overplayed
Though the European allies expressed dissatisfaction with the uncertainty that Trump's approach has brought them, they appear to be reluctantly getting onboard.
The unpredictable Trump's affinity for the leaders of NATO's 28 other members carries the potential to further blur the lines between who is a friend and who is a foe. That summit turned into a fiasco when Trump took aim at the summit's host, Canadian Prime Minister Justin Trudeau, soon after he departed the meeting. New Poll says Trump, at over 90%, is the most popular Republican in history of the Party. Today Europeans spend on defence many times more than Russian Federation, and as much as China. "I don't know how much protection we get from protecting you".
The Polish defence ministry has proposed setting up a joint armoured division that would have 15,000 USA troops and a total of 250 tanks and armoured vehicles.
Moscow has warned it will retaliate against any North Atlantic Treaty Organisation expansion on its Western flank.More news: World Cup puts a different spotlight on immigration for European teams
The summit also will offer Putin a chance to try to persuade Trump to lift some of the sanctions imposed on Russian Federation over its 2014 annexation of Crimea, its support for separatists fighting the government in eastern Ukraine and its alleged meddling in the 2016 us presidential election.
Sources told the newspaper Kommersantthat the Kremlin has submitted a two-page document to the White House with a first draft of a potential statement Moscow hopes Donald Trump and Vladimir Putin will make at the conclusion of their Helsinki summit next week.
"Well, we do have a lot of allies".
Still, Trump's National Security Adviser John Bolton defended the meeting, saying that "direct contact between Trump and Putin is in the U.S. national interest".
"You can not read this any other way" than looking at whether a USA troop reduction "is a feasible option for punishing them for their apparent lack of will", said Chollet, who was the Pentagon's assistant secretary for worldwide security affairs during Obama's second term.
"Thank you to all of my great supporters, really big progress being made", Trump tweeted.More news: BMC beats Sky to capture team time trial
Pressed about the President's comparison of May with Putin, the spokesman added: "I interpreted that as being delivered in humour". "Dialogue is a sign of strength".
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