Will he play nice? That's the question for NATO leaders nervously waiting to see if a carefully choreographed show to please US President Donald Trump will pay dividends when the alliance summit gets down to business on Wednesday.

Every precaution is in place to avoid a Trump blow-up at the Hague summit, from giving him credit for a historic spending deal to keeping the meeting short and sweet.

But that did not stop the volatile US leader launching a pre-summit grenade as he crossed the Atlantic on Air Force One, casting doubt on the very foundation of the 32-member alliance.

He refused to state his commitment to NATO's Article Five clause, the basic agreement that says an attack on one member is an attack on all.

"Depends on your definition. There's numerous definitions of Article Five," Trump told journalists in comments sure to rattle the European allies he dined with upon arrival Tuesday night.

"I'm committed to being their friend," he said.

In a message probably not designed for public consumption, NATO Secretary General Mark Rutte........

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