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.
"Dear America, appreciate your allies, after all you don't have that many".
In 2014, before Trump was president, North Atlantic Treaty Organisation members agreed to stop cutting their military budgets and set a goal of moving "toward" spending 2 percent of their gross domestic product on their own defense - by 2024.
"I really can't say right now". "I think that getting along with Russian Federation, getting along with China is a good thing, getting along with others is a good thing".
The meeting with Putin is arguably the most attention-drawing leg of the tour, considering the accusations in the USA that Trump owes his current office to the Russian president and that there may have been collusion between the Trump campaign and the Kremlin. He has often railed against the US's key allies, accusing fueling trade deficits with the US.
'The president believes that it is important to have a relationship with Russian Federation so that we can talk about the importance of Russian Federation doing better.
"Frankly, it helps them a lot more than it helps us", he told reporters.
He has also pointed the finger at European Union member states for not spending enough on defence - a long-standing complaint of USA presidents, including Barack Obama.
The NATO alliance has been critical to the USA since its formation in 1949 when it was created in response to the Soviet Union.
"In the months ahead, we will step up our cooperation on military mobility, which is essential to our deterrence and defense". "NATO has not treated us fairly but I think we will work something out".
Europe's biggest economy is on course to spend just 1.24 per cent of GDP on defence in 2018.
The hint of USA withdrawal from Europe, while unlikely, indeed seems to have made many countries hike their defense spending.
The European Union is the top export market for the United States at $501 billion in goods and services in 2016, the latest available figures provided by the Office of the U.S. Trade Representative. "Will they reimburse the US?" tweeted the president.
"Who would think? But the United Kingdom certainly has a - they have a lot of things going on". "He is committed to it". That is why we have agreed today to strengthen EU-NATO ties in crucially important areas.