While U.S. Ambassador to NATO Kay Bailey Hutchison has said that the plan for this year's summit is to focus on strategies to counter Russia, Trump may spend the day berating member countries on defense spending.
Germany is extremely reliant on Russian gas, and the new Nord Stream 2 pipeline in the Baltic Sea, which Germany approved in March, will only increase its dependence.
Before he left Andrews Air Force Base, President Trump reiterated his demand for the U.S. to pay less and other members of the Western military alliance to pay more, and he linked the issue to the EU's trade surplus with the US.
And he vowed not to be "taken advantage" of by the European Union, which he accuses of freeloading by relying on the United States for its defence while restricting U.S. imports into the bloc, the world's biggest market.
At the pre-summit breakfast meeting he launched straight back to his theme.
He said the deal meant that Germany was now "totally controlled" by Moscow.
Trump touched down in Brussels aboard Air Force One hot on the heels of a stern warning from the EU's top official that America should "appreciate" its allies.
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.
"European troops have been fighting shoulder-to-shoulder with American soldiers in Afghanistan; 870 fearless men and women sacrificed their lives, including 40 soldiers from my homeland, Poland", said Mr. Tusk.
Meanwhile, Mr. Trump kept jabbing the North Atlantic Treaty Organisation allies as he headed for Brussels aboard Air Force One.
Asked whether he thought Putin was a friend or foe, Trump said he couldn't say, adding that he sees the Russian leader as a "competitor".
"Disparaging long-standing allies at the same time as talking up the country that has wantonly seized territory and redrawn Europe's borders by military force for the first time since World War Two is disconcerting to say the least", said Boudreau.
The U.S. has opposed the project because of its political implications, but work has begun regardless. "Who would think?" Trump told reporters in Washington before departing.
He even suggested his meeting with Putin on Monday would the "easiest" of his trip in Europe.
Trump erased that timeline as he stepped up his calls for increased defense spending, saying allies should do so "immediately".
Last month, Austrian President Alexander Van der Bellen said that while USA politicians are accusing Europe of being dependent on Russian gas, Washington was force-feeding Europe its liquefied natural gas, which is three times more expensive than natural gas from Russia. "We're not going to put up with it, we cant put up with it", he said.
Mrs May said: "Nato is as vital today as it ever has been and our commitment to it remains steadfast".
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