Nato set for 'robust' talks with wayward Trump

Nato leaders also aim to invite Macedonia to join when its name-change protocols have been completed.

The meeting of the 29 allies and more than 10 other friendly leaders at Nato's new HQ in Brussels comes amid ongoing Russian aggression in Ukraine and Syria and in the teeth of Kremlin opposition to Nato expansion in the Western Balkans.

It also comes amid an EU-US rift on trade and ahead of Trump's tete-a-tete with Russian leader Vladimir Putin in Helsinki next Monday.

Trump himself kept Europe guessing on what he might do prior to landing in the Belgian capital on Tuesday evening with his wife Melania.

His tweets struck a belligerent tone.

"NATO countries must pay MORE, the United States must pay LESS. Very Unfair!", he said on Europe's military expenditure.

"Not fair to the US taxpayer. On top of that we lose $151bn on trade with the European Union," he also said, hinting that the EU should back down on tariffs if they wanted US protection.

He was more cocilitaory in remarks to press before boarding his flight, however.

"We will work it out and all countries will be happy," he said on Nato military spending.

"So I have Nato, I have the UK which is in somewhat turmoil, and I have Putin. Frankly, Putin may be the easiest of them all. Who would think!," he added, referring British cabinet resignations over Brexit.

The spectacle of Trump tongue-lashing Nato allies on spending at the summit would weaken faith in its unity and undermine its Russia-deterrent posture.

The spectacle of Trump refusing to sign the Nato communique, or nullifying it with a tweet, as he did at a meeting of the Group of 7 wealthy nations in Canada last month, would do further damage.

But even worse scenarios cannot be ruled out, such as Trump saying it was OK for Russia to annex Ukraine's Crimea, pulling out US soldiers from Germany, or threatening to quit Nato altogether.

The latter scenario would be a catastrophe for Nato credibility, even though it would also be a bluff, according to Douglas Lute, a former US ambassador to Nato.

"Nato is a treaty obligation which was taken by the nation of the United States. It was ratified by a two-thirds majority [in Congress]," he told The Guardian, a British daily.

"Any structural change to that arrangement would have to go through the same process. So this is not something where any president - president Trump or any president - can simply sign America away," he said.

All that amounted to what Nato head Jens Stoltenberg predicted would be "robust … direct and frank discussions" with Trump at Wednesday and Thursday's summit.

He noted that "differences" with Trump also included his views on climate change and on a nuclear arms control pact with Iran.

But he said the alliance had survived other splits before, listing the Suez Crisis in 1956, France's decision to leave Nato in 1966, and the US-UK invasion of Iraq in 2003.

"Nato has continued to be the cornerstone of transatlantic security despite these disagreements," he told press in Brussels on Tuesday.

He added, in a media briefing dominated by US questions, that eight allies were now spending 2 percent, or nearly 2 percent, of GDP on defence.

He said US forces in Europe were "a platform to project [US] power in the Middle East and Africa" as well as to protect allies.

He also said it was "right and proper" to have a dialogue with Putin "at times of high tension", noting that a Nato-Russia council had met seven times in the past two years.

Frank and direct

The European Council head, Donald Tusk, was already "frank" in his remarks earlier the same day, however.

He noted that 870 Nato soldiers had lost their lives fighting alongside the US in Afghanistan since 2001, when Nato sprung to America's defence after 9/11.

"Dear Mr President [Trump], please remember about this tomorrow when we meet ... but above all when you meet president Putin," he said.

"It is always worth knowing who is your strategic friend and who is your strategic problem," he added, in remarks which prompted Trump's "United States must pay LESS" tweet.

The European Commission head, Jean-Claude Juncker, pledged to be a bit more "timid" in his approach, amid the risk that some EU leaders might publicly clash with their US visitor.

Germany, for one, showed no sign of boosting its defence spending, with German president Frank-Walter Steinmeier saying his nation would "drive to the summit with self-confidence", despite not having met the 2 percent target.

"We Europeans need to take more responsibility for ourselves … We have a changed security situation in Europe [and] we are experiencing changes in the European-American relationship," he added.

But Polish leader Mateusz Morawiecki is likely to make nice, amid appeals for a permanent US military base in Poland, even if that meant moving American troops out of Germany to its eastern neighbour.

"We're striving for a permanent US troop presence," Polish defence minister Marius Blaszczak said on Tuesday.

Friends of Trump

Trump's anti-immigration, Russia-friendly, and anti-EU rhetoric is also likely to chime well with other European populists, not least those in Hungary and Italy.

His visit to London, on Thursday and Friday, prior to Helsinki, could also see him wade in to Britain's Brexit "turmoil" after he recently suggested to France that it should leave the EU.

He kept a measured tone on Tuesday, but voiced sympathy for anti-EU hardliners, such as former foreign minister Boris Johnson, who had just resigned in a rebellion over the British government's plans to maintain close EU trade ties.

"Boris Johnson's a friend of mine, he's been very nice to me, very supportive and maybe I'll speak to him when I get over there. I like Boris Johnson. I've always liked him," Trump said.

"That's up to the [British] people, not up to me," he added, when asked if British prime minister Theresa May should be replaced.