As ever with Donald Trump it may simply be bluff and bluster or then again be deadly serious.

One thing is certain though, as this week’s crucial Nato summit in Brussels approaches, many observers are increasingly nervous that the US president is about to seriously undermine the organisation that has been at the heart of Western security and defence since 1949.

It’s no secret that Mr Trump has little time for multilateral organisations and arrangements such as the European Union (EU), the North American Free Trade Agreement (NAFTA), the World Trade Organisation and the United Nations itself.

Nato though has been in Mr Trump’s crosshairs for some time, with the US president arguing persistently that too many Nato countries don’t pay their fair share of defence costs. That much he reiterated again last Thursday night at his MAGA rally in Montana.

“We’re paying anywhere from 70-80% 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, “ Mr Trump told supporters.

Few doubt too that the US president has something of a personal antipathy towards many of the 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.

Earlier last week White House spokesman Hogan Gidley gave a flavour of what the president was likely to tell these key leaders of Nato countries in Brussels tomorrow.

“What the president is going to do is go into these meetings with the mindset to protect the American people, stand with our partners and allies, but as he has said many times before America is thought so often to be the world’s piggy bank. And that’s gotta stop,” Gidley stressed.

Provocative as such a demand would likely be, Mr Trump looks set to compound the acrimony by following up with a cosy engagement with Russian President Vladimir Putin a few days later in Helsinki. Mr Putin’s primary goal for so long now has of course been to sow divisions within the Nato alliance.

Any scheduled Trump - Putin meeting would fit with the US president’s slash and burn strategy of a few weeks ago when he rattled the G-7 gathering in Quebec, only then to go on and buddy up with North Korean dictator Kim Jong Un at talks in Singapore.

To be fair to Mr Trump he is not the first US president to complain about the lack of burden-sharing by Nato member countries. Many too would argue that the Europeans should contribute more. Right now the totemic figure of 2% of GDP is the established lower threshold member states should aim for.

But while some members like the UK and smaller counterparts like Greece meet the 2% target, other larger members like Germany fall short. Right now Germany spends about 1.2% of GDP on defence and aims to boost such outlays to 1.5% by 2024.

Mrs Merkel has riled Mr 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 Mr 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 Wednesday’s crucial and potentially volatile summit looms?

Right now even senior American officials said they have no clarity on Mr Trump’s intentions for the meeting.

But the Trump administration has already reportedly been analysing a large-scale withdrawal of American forces from Germany, after Mr Trump expressed surprise that 35,000 active-duty troops are stationed there.

Hardly surprising then that the talk in some Nato circles now is that Mr Trump could fatally undermine the alliance should he pursue such a withdrawal. There are fears too that he may even storm out of the summit or cast doubt on the US nuclear umbrella over western Europe.

Any such drastic measures would of course seriously undermine the pledge at the heart of Nato that an armed attack on any one of its 29 members (originally 12) 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, the famous Article Five was a near unprecedented pooling of sovereign power on a scale never before seen. All that now could be under threat. Then again Mr Trump is well know for barking but not always biting when it comes to issuing such threats.

Widely regarded as one of the most successful and effective international institutions of the 20th century, Nato faces a rocky time this week. Mr Trump is also not the only problem the alliance faces. The thorny issue of Turkey, hitherto Nato’s staunchest member, with one of its largest military forces is also pressing.

Newly re-elected president, Recep Tayyip Erdogan, seems to be wavering about his commitment to the West. Reported to be already buying aircraft and missiles from the Russians, questions are increasingly asked about just whose side Mr Erdogan is on.

Certainly Turkey’s departure from Nato would be a colossal blow and throw even more fuel onto an already combustible situation.

Many in Europe already see Mr Trump as determined to mischief make, perhaps to the point of causing serious damage to the foundations of the US-led international order of which Nato is part. Tea with the Queen might be the most sedate of meetings during Mr Trump’s visit to Europe this week. For the leaders of the Nato countries, their meeting with Mr Trump could prove a much more toxic brew and will be a closely watched test of the US president’s commitment to this historic alliance.