Twitter is his favourite method of admonishing people. That much was again evident earlier this month when US president Donald Trump fired off a series of internet missives that marred the summit meeting of G-7 nations in Quebec.

This week, as a crucial Nato summit in Brussels looms, there are already fears that Trump is about to do the same, or much worse, with America’s Nato "allies".

In many respects Trump’s onslaught on Nato has already begun. As early as last month, long before this Wednesday’s Nato summit was due to start, he was already on the offensive firing off scathing letters admonishing Germany, Belgium, Norway, Canada, and others.

In the correspondence he warned them that the US is losing patience with the countries' failure to meet their financial commitment to their security obligations under the Nato alliance. The letter sent to German Chancellor Angela Merkel was particularly pointed.

“As we discussed during your visit in April, there is growing frustration in the United States that some allies have not stepped up as promised,” Trump wrote to the German leader.

“The United States continues to devote more resources to the defence of Europe when the Continent’s economy, including Germany’s, are doing well and security challenges abound. This is no longer sustainable for us,” Trump added.

Since then his pressure on the leaders of the Nato countries he feels are “taking advantage” of Washington has been unrelenting. That much was clear again last Thursday night at his MAGA rally in Montana where Trump pulled no punches on what he sees as other Nato members “freeloading”.

“We’re paying anywhere from 70-80 per cent to protect Europe …of course they kill us on trade, they kill us on other things… and we’re the schmucks paying for the whole thing," Trump told supporters.

“I’m going to tell Nato, you gotta start paying your bills. The United States is not going to take care of everything,” Trump warned ahead of the summit.

The president’s blunt missives and scathing remarks have been met with a mix of hand-wringing and outrage both in the US and abroad. It’s hardly surprising that there is now a pervasive sense of foreboding among many Europeans and some Americans that Trump may be about to seriously undermine the organisation that has been at the heart of Western security and defence since 1949.

Germany, the Nato member for whom Trump has singled out his most scathing criticism, appears in turn to be losing patience with the US President. Some leading German politicians and officials over the last few days are even now openly questioning his loyalty to Nato.

Christian Lindner, head of the pro-business Free Democrats, told German public broadcasting radio station Deutschlandfunk in an interview that he did not trust Trump.

“He is too volatile...Within 24 hours, Mr. Trump can change his position by 180 degrees,” said Lindner, although he warned against growing anti-American sentiments in Germany, given that the United States was and would remain Germany's closest ally.

Wolfgang Ischinger, head of the Munich Security Conference and a former German envoy to Washington, said it is possible that Trump could refuse to sign a communique at Wednesday’s summit, mirroring what he did at the recent G-7 gathering in Canada. It was at that same G-7 summit of course that Trump was reported to have said that, “Nato is as bad as Nafta.”

It has long been no secret that Trump has little time for multilateral organisations and arrangements such as the European Union, the North American Free Trade Agreement, the World Trade Organisation and the United Nations itself.

But his potential to do serious damage to Nato worries many not least because it comes at a time when Russia has been reasserting itself militarily on the world stage. Nato members fears are further compounded by the fact that following Wednesday’s summit, Trump is set to have a one-on-one meeting with Russian President Vladimir Putin in Helsinki.

The concern is that not only might the impetuous and unpredictable US President lay bare Nato’s schisms but also publicly embrace Putin, a leader with whom Trump seems to have a special empathy, despite Moscow’s determination to sow divisions within the Nato alliance.

Peter Beyer, transatlantic coordinator for Merkel's ruling coalition in Germany, was quoted this weekend in newspapers as saying he is concerned that Nato states were not included in the planning for Trump’s meeting with Putin.

“There are great concerns in the alliance about what agreements Trump and Putin could reach," Beyer told the Funke Mediengruppe.

In unusually frank remarks he told the media group that Trump's recent summit with North Korean leader Kim Jong Un in Singapore has fuelled concerns that Trump would let Putin “put one over on him” in Helsinki.

Trump’s cosy relationship with Putin is a far cry from the personal antipathy he has toward many key leaders of Nato countries. His open dislike of Germany’s Angela Merkel, Canada’s Justin Trudeau, and Prime Minister Theresa May appears increasingly to many diplomatic watchers as deep rooted and intractable. Even the brief flash of bonhomie toward President Emanuel Macron of France during an April visit to Washington has cooled considerably since.

So what then exactly lies at the core of Trump’s gripe with Nato beyond the American president’s obvious suspicions and personal dislike of certain individuals and multilateral organisations?

The short answer is money, or what Trump sees as the lack of burden sharing by Nato member countries.

To be fair to Trump he is not the first US president to make such complaints. Many too would argue that the Europeans should contribute more.

As for reaching the two per cent of gross domestic product on their military budgets that was agreed during the 2014 summit in Wales, progress has been slow but steady. On average, Canada and the European nations have reached 1.45 per cent of GDP compared with 3.57 per cent spent by the United States.

But while some members like the UK, and smaller counterparts like Greece meet the two per cent target, other larger members like Germany fall short. Right now Germany spends about 1.2 per cent of GDP on defence and only aims to boost such outlays to 1.5 per cent by 2024.

Chancellor Merkel has also riled Trump further by insisting on looking beyond purely military expenditure when considering the Nato target. She argues that because development aid is vital to security, it should included in contributions toward the common defence.

Problematic as these differences are, they are made all the more corrosive by the fact that Trump is urging more than a burden shift. Actively he has linked his pressure on European allies with threats about trade, seeming to say that unless the Europeans pay more he will tighten protection against European exports to the United States. The car industry in particular has been strongly in his sights, especially BMW and Mercedes.

So where does all this leave Nato as this week’s crucial and potentially volatile summit looms? Right now even senior American officials said they have no clarity on Trump’s intentions for the meeting.

What has emerged is that Trump – not a man known for his grasp of detail – has been "taken aback" by the size of the US military presence in Germany and has demanded an analysis of the impact and cost of withdrawing or redeploying those forces. Currently some 35,000 active-duty US troops are stationed there. As the summit nears however the Trump administration has been keen to play down such reports.

“There is nothing being said at all about the troop alignment in Germany or anything that would change the current basing arrangements there,” the administration’s ambassador to Nato, Kay Bailey Hutchinson, told reporters in a briefing last week.

Such reassurances however have done little to allay the fears of both Pentagon chiefs and State Department diplomats, already nervous that the Trump-Putin Helsinki get together could result in the President impulsively announcing a cancellation or reduction in US involvement in Nato military exercises, similar to the one he produced after his Singapore tête-à-tête with Kim Jong-Un.

The current differences between Trump and Nato stand in marked contrast to that time 69 years ago, when the foreign ministers of 12 countries in North America and Western Europe gathered in Washington, DC, to sign the North Atlantic Treaty in which they “resolved to unite their efforts for collective defence and for the preservation of peace and security”.

The organisation that emerged, the North Atlantic Treaty Organisation (Nato), has since underpinned the longest period of sustained peace and prosperity in the West’s modern history.

Today it is widely regarded as one of the most successful and effective international institutions of recent times, a measure of what might be at stake on Wednesday should Trump throw a spanner in the works.

Should he chose to do so it would seriously undermine the pledge at the heart of Nato that an armed attack on any one of its now 29 members should be considered an attack on all, and that all member states should be ready to assist with armed forces.

In its time the agreement and its Article Five was a near unprecedented pooling of sovereign power on a scale never before seen.

But as Ana Palacio, a former Spanish foreign minister, observed in an article on the online commentary website Project Syndicate last week, the truth is that, as important as Article Five and the two per cent of GDP spending requirement are, Nato’s value and relevance extend far beyond these issues.

“Consider Articles two and three of the Nato Charter. They are rarely referenced, yet both are of paramount importance to fulfilling Nato’s purpose,” Palacio points put.

“Whereas Article Five has become a source of leverage for Trump to use to pressure his allies to spend more, while impressing his domestic supporters, Articles Two and Three are practical and direct. They focus not on visions of war, but on the building blocks of peace, including public education, improved institutional relations, and effective organisation,” observed Palacio.

But far and away it remains security concerns that are the most worrying should Trump decide the time has come to shake up Nato.

“America’s partners are particularly concerned about a surprise Trump giveaway during his meeting with Putin: announcing a withdrawal of significant Americans troops from Europe, cutting defence funds to US European Command, or stopping exercises with Nato’s easternmost members, which Russia protests as provocative,” noted James Stavridis, Nato’s Supreme Allied Commander from 2009 to 2013 last week in a Bloomberg op-ed.

“Given the script he is executing with North Korea – including a pause on military exercises with South Korea that apparently blindsided not only Seoul but also Secretary of Defence James Mattis – these fears would appear very justified,” added Stavridis in an assessment that will only further set nerves on edge in Nato’s ranks.

Trump’s last big sit down with European leaders, at the G7 summit, ended with the instantly iconic photograph of Angela Merkel staring down a defiant Donald Trump, arms resolutely folded and surrounded by frowning leaders doing their best to project grave concern.

This Wednesday could well see a rerun of such a showdown. Trump during his real estate career has learned that tough talk and bluster cost him nothing and can intimidate his opponents. It remains to be seen if that or a slash and burn strategy is his ultimate aim this week towards Nato. Either way, trust between Washington and its European allies is facing a real test, and Wednesday’s summit could well be the deciding factor in the outcome of that test.