NATO leaders on Wednesday agreed to hike defence spending and expressed their "ironclad commitment" to come to each other's aid if attacked,a day after US President Donald Trump appeared to cast doubt on the strength of the alliances core security guarantee, saying it had "numerous definitions".

NATOscardinal principle that an attack on one member is an attack on all came under renewed scrutiny on Wednesday as US President Donald Trump once againcastdoubton how it should be understood.

Trumps remarks, made aboard Air Force One on the eve of a NATO summit in The Hague, reignited long-standing fears among European allies that the US may no longerbe committed to the principle enshrined in Article 5 of the alliance's founding treaty.

"There's numerous definitions of Article 5," Trump said. "You know that, right? But Im committed to being their friend."

Read moreTrumps wavering on mutual defence clause overshadows shortened NATO summit

Asked again on Wednesday about the clause, Trump appeared to strike a softer tone, telling reporters: "Were with them all the way." But the ambiguity left many NATO members unconvinced.

NATO Secretary General Mark Rutte responded quickly, saying he had "no doubt" about the US commitment to the alliance.

"For me, there is absolute clarity that the United States is totally committed to NATO, totally committed to Article 5," Rutte told reporters before the start of the summit.

A fragile cornerstone

Article 5 of theNorth Atlantic Treaty, which was signed in 1949, states that any armed attack against one or more NATO members will be considered an "attack against all members".

Each country agrees to take "such action as it deems necessary", including the use of armed force, to restore and maintain security in the North Atlantic area, the article adds.

The clause was designed to deter Soviet aggression in Europe and was never triggered during the Cold War. It was invokedonly once, after the September 11, 2001 attacks, when NATO supported the US-led intervention in Afghanistan.

TheUShas historically resisted language that would make the collective response automatic.

"This vague wording leaves room for individual assessment and helps secure consensus among allies," said Olivier Kempf, a senior fellow at the Foundation of the Strategic Research.

"When Trump says there are many interpretations of Article 5, hes technically right. Its a leap of faith. It only works because members trust each other."

Undermining trust

Trump has long criticised NATO, accusing European countries of free riding on US security guarantees. In March, he again questioned whether European allies would defend the US if roles were reversed.

"If the United States was in trouble and we called them. We said, 'We got a problem, France. We got a problem. A couple of others, I won't mention. Do you think they're gonna come and protect us? They're supposed to. I'm not so sure," he said during a campaign rally.

French President Emmanuel Macronhit backat the time, insisting France was a loyal and steadfast ally and had every right to expect reciprocity.

This week, Trumpdoubled downon demands for increased defence spending. He called the current 2% GDP target insufficient and pushed for a new threshold of 5%, including broader security spending.

"NATO was broke," he said. "Then I said, Youre going to have to lift it to 4% or 5%, and 5% is better.'"

'A great victory'

Pressure from Trump appeared to pay off at the summit in The Hague as Washington's NATO allies dulyagreedto massively hike defence spending to 5% of GDP by 2035.

Read moreTrump hails NATO spending pledge as 'monumental win for US'

For Europe's leaders, it was the price to pay to ensure Trump would endorse a final declaration reaffirming NATO members' "ironclad commitment"to mutual defence.

"The Europeans will sign anything, even if their insurance premium just doubled, because the world is more dangerous now," Kempf explained.

"I think we're witnessing the birth of a new NATO, which means a more balanced NATO and a NATO which has more European responsibility," said Finnish President AlexanderStubb, whose country recently joined the alliance due to the increased threat from Russia.

Trump appeared satisfied with the outcome. "This is a great victory for everyone," he said of the new defence spending deal, which he hailed as a "monumental win" for the US.

And while doubts remain over his commitment, his closing remark "I stand with Article 5" may offer just enough reassurance to keep NATOs cornerstone intact. For now, the alliances leap of faith appears to be holding.

Originally published on France24