The timing alone has already cast a dark symbolic shadow at a moment when a clear display of consensus is critical for Western unity.

Some European diplomats I spoke with in Washington ahead of the summit fear Trump will do more than just denigrate Nato, going so far as to threaten a US withdrawal from the military alliance before engaging in a lovefest with Putin that will enable the Russian leader to further exploit Washington’s differences with Europe.

In the end, Trump claimed victory after the Nato summit, claiming progress had been made on defence spending only hours after throwing the Brussels meeting into disarray with fresh attacks on European allies.

Asked whether he had threatened to pull out of Nato, he told a press conference before he was scheduled to leave that he only told people he would be “extremely unhappy” if spending was not increased.

Add to that issues such as a looming trade war and Trump’s desire to have Moscow readmitted to the G8 (reduced to the G7 after Russia’s 2014 annexation of Crimea), uncertainty about where the Trump administration is ultimately heading is shaking a continent swept up in an anti-immigrant, right-wing populist tide.

More worried than angry, most European leaders have so far been loath to provoke Trump with open criticism. They fear the summit with Putin will deal their relations with Washington another blow by selling out Western security.

Beyond the symbolic meeting of minds, Putin could persuade Trump to halt US-led Nato military exercises in Poland and the Baltic states that Russia vehemently opposes, and try to ease US sanctions on Russia without addressing Crimea. Either would be devastating for Western interests.

Events are moving fast. Trump clashed with German chancellor Angela Merkel on Wednesday, telling her that Germany was “totally controlled by Russia” because it imports high levels of Russian natural gas.

Earlier, he had directed curt letters to her and leaders of other Nato allies, repeating his criticism that they spend too little on their own defence. For his part, Nato chief Jens Stoltenberg has warned that the transatlantic relationship and its military alliance may not ultimately survive.

Trump’s critics do not dispute that under normal circumstances, the leaders of the two largest nuclear powers should be talking to each other directly about issues such as nuclear accords and arms limitations.

But few foreign policy observers in Washington believe Trump will advance Western security by engaging Putin on those pressing issues — including an extension to the New START arms reduction treaty between the US and Russia, which expires in three years — or confront Moscow about destabilising Ukraine, propping up Syria’s murderous Bashar al-Assad, and meddling in the 2016 US presidential election.

Trump publicly disagrees with the US intelligence community and members of his own administration who say Moscow interfered in the ballot on his behalf.

Russia “continues to say they had nothing to do with Meddling in our Election!” he tweeted last month, adding that he hasn’t ruled out recognising Russia’s illegal annexation of Crimea.

With Putin’s perceived role as America’s main geopolitical adversary central to his all-important domestic popularity ratings, he will almost certainly use the summit to burnish that image.

Trump will presumably help validate Putin’s moves to undermine the US-led liberal world order. In previous meetings with Putin, foreign minister Sergei Lavrov, and other Russian officials Trump admires, he has already shown his weakness for flattery — something Putin, the ruthless former KGB officer, will doubtless try to exploit.

Whatever happens, the summit will almost certainly deepen the rift between the US and its European allies.

Their dilemma is that Trump’s relentless antagonism towards them is driven not by their failure to support the US-led transatlantic alliance, but their insistence on preserving it.

They know responding in kind would only reinforce his attack on Western unity. At some point, however, they will grow weary of rolling with his punches.

Some officials already have. On Tuesday, European Council president Donald Tusk noted the EU spent more on defence than Russia and as much as China. “Appreciate your allies,” he said in a message to Washington after the signing of a joint EU-Nato declaration. “After all, you don’t have that many.”

Gregory Feifer is executive director of the Institute of Current World Affairs and author of Russians: The People Behind the Power. He is a member of a bipartisan working group in Washington on European democracy that is urging Nato to reaffirm the North Atlantic Treaty, with its principles of “democracy, individual liberty, and the rule of law”