He triggered an emergency session on the spending issue, before emerging from the turmoil to say "I believe in Nato" in response to reports that he had threatened to pull out.
This alliance of 28 countries from around the world is a pillar of American interests since its inception many decades ago.
"They're here as a prime minister or as a president, and they can't necessarily go in and say, 'This is what we're going to do.' But they're going back for approvals", Trump said, acknowledging the political realities.
Trump has said next week's meeting with Putin "may be the easiest" part of his European tour. But I would also encourage the president to exercise caution with Russian Federation - which has proven to be a hostile nation to the United States and our Western allies.
"Germany has agreed to do a lot better than they were doing, and we're very happy with that", Trump said, without going into specifics.
However, what really happened was that the US president retained an insulting and aggressive persona when the world was watching; a move that some believe Trump may be trying to pull to appeal to his right-leaning political base.
It's a typically Donald Trump approach to diplomacy and getting things done - he likes to sign pieces of paper that are largely symbolic, he attacks allies on Twitter but keeps sweet in person - and he sabre-rattles using the gravitas of the United States to get his way.
And while the president has said he ultimately wants to up commitments doubled to 4 percent, for now he said that countries have agreed to move at a "faster clip" to meet the 2 percent threshold.
He demanded that North Atlantic Treaty Organisation countries "Must pay 2% of GDP IMMEDIATELY, not by 2025" and then rattled them further by privately suggesting member nations should spend four percent of their gross domestic product on defence, a bigger share than even the United States now pays, according to North Atlantic Treaty Organisation statistics. "We have a tremendous relationship with Germany", he said. "It confirms the goal of 2% by 2024".
Trump's assertion that Russia is a competitor and not an adversary is contrary to the judgments of USA intelligence agencies and his chief diplomat, Mike Pompeo, who at his April confirmation hearing to be Secretary of State, said the United States needs "to push back in each place we confront them", referring to the Russians.
Trump stressed that NATO's budget had been unfair to the USA but now he was sure it would be fair.
A march in support of Trump will start at the U.S. Embassy on the south bank of the River Thames and end near the prime minister's residence at Downing Street on Saturday.
The outdoor arrival ceremony at Blenheim - Trump wore a tuxedo and first lady Melania Trump a butter-yellow, chiffon off-the-shoulder gown - was a grand affair marked by a military band in bearskin hats, hundreds of business leaders in black tie and gorgeous setting sunlight.
The latest tariffs Trump has proposed, on $200 billion worth of Chinese-made goods, would raise the price of 40 percent of China's exports to the USA and begin to shrink the $375 billion trade surplus Beijing ran in 2017.
He said he viewed Putin as a "competitor, ' but not as a 'friend" or an 'enemy'.
Most nations are reluctant to commit major portions of their GDP annually to buying these pricey U.S. arms, however.
Trump said his summit with Putin, which will take place on Monday in Helsinki, would be a loose meeting with "no big schedule" - but one that could still be productive.