He also publicly thrashed White House Chief Strategist Steve Bannon, declaring, "I'm my own strategist" and suggesting that the man he once said he has known "for many years" was just "a guy who works for me".
But Trump's about-face has more to do with those closest to him and the way Stoltenberg has focused on shared priorities with the new President, as well as the deterioration of another key relationship: that of the U.S. and Russian Federation.
The secretary general refused to take the bait, explaining that he "welcome [s] his very strong message on defense spending, on burden sharing, and also on NATO's role in fighting terrorism". "But we're going to see what happens", Trump told the news conference.
"I'll be speaking with Rex Tillerson in a little while".
What we see as flip-floppery and weakness, Trump sees as flexibility and uncertainty - which he thinks are two hallmarks of any strong deal-maker.
Yesterday, however, Mr Trump said he had growing concerns about Russia's support of Syrian President Bashar al-Assad.
Blitzer tried to push back, eventually asking Stoltenberg if he trusted President Trump.
The point is that Trump took these positions not because they were long and deeply-held views but because they made sense for him politically at the time.
The Trump administration itself is now under FBI investigation for its links to Russian Federation, and for its possible role in working with Moscow's effort to interfere in the 2016 U.S. election to benefit Trump. A dubious, if not unsafe claim, not only has that not happened almost four months into his presidency, Trump recently wrote the idea off completely.
Embattled O'Reilly takes his longest spring break in years
MSNBC was second with 1.6 million viewers, followed by USA Network's 1.5 million viewers and HGTV's 1.4 million watchers. The company declined to comment Wednesday.
On Wednesday, following his meeting with Stoltenberg, Trump, who had been a frequent critic of North Atlantic Treaty Organisation, declared the alliance "no longer obsolete" and called on its 29 member countries to join together against Syrian President Bashar al-Assad and to fight terrorism - but he repeated his campaign demand that North Atlantic Treaty Organisation "pay what they owe" to the United States.
Hours before the event, a US official agreed that better burden sharing is "on the right trajectory", noting that for the first time in 2015 and, after a collective decline in allies' national defense budgets, the numbers "started to peak back up".
By contrast, Trump was full of praise for Xi Jinping - whom he met in Florida last week - saying that he believed the Chinese President was honest in helping defuse the USA showdown with nuclear North Korea.
The US launched an airstrike on Syria in response to the street and Mr Trump has said relations between the two countries are at an "all-time low". "I felt pretty strongly that they had a tremendous power (over) North Korea".
During the presidential campaign, Trump repeatedly slammed the members - 23 out of 28 - that are not meeting the alliance's recommended defense spending levels of 2% of GDP.
Trump referred to the Syrian leader as a "butcher".
The reversals come on the heels of Trump launching a missile attack on Syria, after spending years complaining that the United States should stay out of conflicts in the Middle East, and should avoid attacking Syria in particular. "We have to find ways to live with them and to try and avoid a new Cold War, a new arms race", Stoltenberg said.