Despite his controversial comments, the Bundestag President declared Mr Trump was "not entirely wrong" about forcing member states to up their defence, saying that he can "understand that critical position".
Trump plugged three top US defense contractors in a televised press conference, and said he told less wealthy nations he would "help them out" in procurement of USA -made products and pitched major USA firms.
President Trump has arrived in the United Kingdom for a much-anticipated visit that is expected to draw large protests.
Ahead of his Monday meeting with Putin in Helsinki, Trump called the Russian president a "competitor" when asked if he considered Putin a friend or a foe.
Woody Johnson, the U.S. ambassador to the United Kingdom, dismissed the significance of the protests, telling Fox News that one of the reasons the two countries are so close "is because we have the freedoms that we've all fought for".
Mr Trump's early-morning tweets from Brussels had been typically critical. But not before Merkel and other top officials in the German government had publicly fired back at Trump's breakfast diatribe. "He seems to look at this as we do everything for them, and we provide them with benefits, and they just take advantage of us". Trump has repeatedly denounced low levels of defense spending among remaining members, particularly Germany.
Trump closed out the tumultuous two-day summit by reaffirming the United States' commitment to the almost 70-year-old alliance and signing the joint declaration, despite spending much of the summit berating allies over defence spending and accusing Germany of being beholden to Russia because it buys Russian natural gas. Cory Gardner, Colorado Republican, said Thursday that he believes Russian Federation is worthy of being designated a state sponsor of terrorism. The communique welcomes "four consecutive years of real growth in non-US defence expenditure" and says all allies have started to increase the amount they spend on defence in real terms. "That's very unfortunate", said Stavridis. "It confirms the goal of 2 percent by 2024".
At the Thursday press conference, Trump claimed that his push for a focus on defense spending had helped make North Atlantic Treaty Organisation a stronger alliance. "The additional money they will be putting up has been really fantastic".
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Trump then held an impromptu press conference.
"I am very glad that we are united today in freedom as the Federal Republic of Germany and that we can therefore also make our own independent policies and make our own independent decisions". "I'm a very stable genius".
Trump leaves Brussels on Thursday for some extra time with one key member of the alliance: British Prime Minister Theresa May.
"The clear message from President Trump is having an impact", he said.
Trump tweets: "Why are their (sic) only 5 out of 29 countries that have met their commitment?"
White House trade adviser Peter Navarro will lead a high-level team attending the Farnborough Airshow outside London next week to advocate on behalf of USA aerospace and defence companies.
Trump's trip to Britain will test the bonds of the USA and U.K.'s special relationship.
He also said meeting the queen would be an experience Trump "will really cherish". "Because I said it was unfair".