"It's no longer obsolete", Trump told reporters at a joint White House news conference with NATO Secretary General Jens Stoltenberg.
Mr Trump, a former real estate developer, took office in January as a government novice whose foreign policy mantra was a vow to keep the U.S. safe and build up the USA military.
Such an admission raises the same question proposed by The Washington Post's Dana Milbank: Is the president "discovering for himself things the rest of us already knew?" The NATO official said Trump was "right". After all, the President has spent most of his first 100 days in office torching conventional political practice, trading in untruths and exaggerations, and pouring oil on political controversies on Twitter - including accusations that his campaign had links to the Kremlin at a time when Moscow was being accused of interfering in the United States election.
Stoltenberg said at the news conference: "In NATO, America has the best friends and the best allies in the world".
He only resigned after a barrage of negative press after it came to light that he received millions of dollars in undisclosed payments from the party of the Russia-backed Ukrainian leader, Viktor Yanukovich. We've a very strong country.
President Donald Trump has a new best frenemy. The White House, however, is struggling to explain some of the changes.
Trump has also been turning to outside business leaders, including many he's known for years, for guidance. Apparently now it seems they don't have that option.
Despite initial hopes in Moscow of better ties with Washington under Trump, the USA new president has tempered his praise of Putin, most recently condemning his support for Syria's Bashar al-Assad.
The U.S. responded with force to the chemical weapons attack that killed scores of victims in the Syrian town of Khan Shaykhun. I'm just hoping that dialogue, cool heads, and adult supervision will prevail here, " she added. That's an institution that hasn't changed at all.
American foreign policy may be returning to customary, well-worn tracks.
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"I said it was obsolete". Now he says it is not obsolete, " he said. "This is exactly the opposite of the positions he took consistently during his campaign". Trump doesn't feel any allegiance to past positions or, really, past statements about issues.
"The other major topic will be fair burden-sharing in our Alliance".
However, all 28 allies decided in December 2014 to gradually increase spending on defense. "It is time to end this brutal civil war, defeat terrorists, and allow refugees to return home", he said.
Sandy Pho, a senior program associate for the Kissinger Institute on China at the Wilson Center, said that Trump, like many new presidents, has been facing a learning curve on the ways of Beijing.
As recently as January, Trump continued to stand by this position - which alarmed many North Atlantic Treaty Organisation members - saying in a Jan. 15 interview with the Times of London and Germany's Bild that North Atlantic Treaty Organisation is "obsolete because it wasn't taking care of terror" and that critics of his comments have "started saying Trump is right".
Stoltenberg said he strongly believes that the only way to deter Russian Federation is to be strong. This includes very small countries: "Romania, which although is a large country geographically, has a very weak economic base; and Latvia, which is a very small country indeed", he explained. We see this commitment not only in words, but also in deeds. "We'll see whether or not he does".
Trump is expected to travel to Brussels later this spring to attend the NATO Summit. "It is not going to happen", he said.
President Trump is saying relations with Russian Federation "may be at an all-time low", just hours after Secretary of State Rex Tillerson held a lengthy meeting with President Vladimir Putin, with the two superpowers still not seeing eye to eye on the cause of last week's chemical attacks in Syria or how to deal with dictator Bashar Assad.
"The signs again in the short term are very pessimistic, unfortunately". This will only have the opposite effect. Because Russia will not go away. "And our country hasn't had a clue, they haven't had a clue".
Stoltenberg also praised Trump for putting pressure on alliance members to fulfill their obligations by increasing defense spending.