(CNN) President Donald Trump begins a six-day European trip amid blazing anxiety among US allies over his commitment to the transatlantic alliance and antipathy for its leaders and institutions while stirring new disquiet about his cozy, baffling relationship with Russian President Vladimir Putin.

Such concerns will color a tense NATO summit in Brussels, Belgium, beginning Wednesday; Trump's long-delayed visit to Britain, where turmoil over Brexit is raging; and his first standalone summit with Putin in Finland on Monday.

Trump wasted no time, on Wednesday accusing fellow NATO ally Germany of being beholden to Russia because it buys energy from Moscow, in pointed remarks ahead of a summit of the military alliance in Brussels

"Germany is a captive of Russia," Trump said at a meeting with NATO Secretary General Jens Stoltenberg, his first since arriving in the Belgian capital. "It's very inappropriate."

Trump also complained that the United States is expected to "defend them against Russia," despite Germany making "billions of dollars" in energy payments to Moscow. "I think it's something that NATO has to look at," Trump said.

Stoltenberg pushed back on Trump's comments, saying that while there may be differences of opinion within the alliance, "we are stronger together than apart."

But for the last 17 months, Trump has torn at the West's cohesion and questioned its values in a startling manner since, typically, the US has always seen European institutions as multiples of its own power and enhancing its own security.

He's portrayed US allies as freeloaders exploiting American generosity rather than partners in a US effort to rebuild shattered Europe after World War II and an alliance that beat communism in the Cold War in a triumph for liberal democratic capitalism.

It's an assault that has opened wide divides in the transatlantic alliance and plays directly into what US intelligence agencies and foreign powers assess as Putin's goal -- to cement his own autocratic rule by weakening the institutions of the West.

As it is, transatlantic relations are in their worst state in 70 years, as an increasingly unfettered US President acts on his populist nationalist intuition, inciting a trade war with the European Union and parroting the foreign policy talking points of the Russian strongman he admires

"The question everybody has here is what is the world going to look like after this couple of days here? Is an already undermined system getting a further blow?" said Thomas Kleine-Brockhoff, a former German presidential adviser, speaking from Brussels.

Before leaving the White House, Trump showed why there is such concern in Europe, a month after he blew up the G7 summit in Canada.

"NATO countries must pay MORE, the United States must pay LESS. Very Unfair!" Trump tweeted.

NATO countries must pay MORE, the United States must pay LESS. Very Unfair! — Donald J. Trump (@realDonaldTrump) July 10, 2018

Then Trump told reporters his meeting with Putin would be easier than those with US allies , further stoking worries that his hostility to NATO could embolden Russia.

The comment was yet another example of Trump siding with a leader seen by US allies as a threat to democracy, who US spy agencies say meddled in the 2016 US election to help Trump win and who presides over a security state likely responsible for the death of a British woman after a nerve gas attack on a Russian former spy on UK soil.

Trump's defenders say his repeated demands for NATO nations to reach the threshold of 2% of their gross domestic product spent on defense are amplifications of his predecessor's demands for more burden sharing -- and they have a point. There's an argument that by forcing increased defense spending, Trump is actually strengthening NATO.

Since the end of the Cold War, European governments have slashed military spending as they struggle to finance welfare states, while the lack of an existential threat to the West has eaten into military readiness.

But the President's false claims that US allies owe billions of dollars in unpaid dues undercut alliance unity.

He ignores the fact that the only time NATO's Article 5 creed on collective defense has been invoked was to help the US after the September 11 terrorist attacks in 2001 or that US allies shed considerable blood in US wars in Afghanistan and Iraq.

"He is definitely weakening the alliance. The question is whether it will survive his presidency or not. Part of that will depend on how long he stays in office," Max Boot, a historian and CNN national security analyst, told CNN's Wolf Blitzer on Monday.

How will the NATO summit play out?

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Trump is nothing if not unpredictable and his tough talk could be a prelude to a declaration of victory over more burden sharing.

Partly as a result of Trump's complaints -- as well as a rise in the perceived threat from Russia -- every US NATO ally is spending more on defense, as a 2024 deadline looms for military budgets to hit 2% of GDP.

"We will talk about the biggest increase in defense spending by our allies since the Cold War," US Ambassador to NATO Kay Bailey Hutchison said last week in a briefing that highlighted "malign" threats from Russia.

"The major overall theme of this summit is going to be NATO's strength and unity," Hutchison said, like other Trump team members who back conventional US positions but appear to operate in a parallel reality to the President's instinct-driven policy.

If Trump celebrates increased spending, gently urges laggards like Germany to do more and avoids a confrontation, NATO may escape significant damage this week.

But such a scenario appears to belie everything that he's long believed about allies exploiting the US and his distaste for multinational approaches to common threats that contradict his "America First" philosophy.

If that mistrust spills over, it would play directly into Russia's hands.

"If that happens, I think the alliance faces a crisis of historic proportions," former US Ambassador to NATO Ivo Daalder said on CNN's "New Day" on Saturday.

Whatever happens this week, Kleine-Brockhoff believes that Trump has already done lasting harm to the Western alliance.

"An alliance consists of hardware and software. Hardware is tanks and planes and ships. Software is resolve and unity. If you don't have those two, you cannot deter," the former German presidential adviser said.

"The belief of others that this is an alliance, that it will act in unison and it will act when challenged is already undermined. That damage is already here," said Kleine-Brockhoff, now with the German Marshall Fund.

Some key Americans also worry that Trump is hurting NATO.

"I'm very uncomfortable with it," Sen. Bob Corker, a Tennessee Republican who's the chairman of the Senate Foreign Relations Committee, said Tuesday.

"The destabilizing effect it is having in that region is significant."

Trump not only to blame

It would be wrong to blame Trump for all of the turmoil sweeping Europe that is challenging the institutions that underpin Western security.

In recent years, the continent has battled financial crises and been thrown into turmoil by debates over immigration that have also rocked US politics.

With the World War II generation dying off and memories of the Cold War receding, European nations are well overdue for a rethink of the purpose of the West, since an American reassessment was always likely, with or without Trump.

Europe is also struggling with the explosion of populism that predated Trump's insurgent White House race in 2016. And the continent's leaders are coming to realize that China's fast-growing might could challenge its relative power.

Such challenges might seem good reasons to strengthen the transatlantic alliance.

Yet there is a strong feeling in Europe that Trump is doing the exact opposite and has little feeling for values like free expression, diversity and liberal democracy that are at the core of Western civilization.

Long-held views

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Years before he ran for the presidency, Trump made no secret of his transactional view of US alliances, seeing them more in financial terms than as a way to project US power and common values.

His rejection of the Paris climate pact and the Iran nuclear deal defied European entreaties, and left an impression that the region's national security priorities were simply collateral damage to the President's quest to solidify his political base.

And Trump clearly has more of an affinity to the forces disrupting the status quo than to leaders like Merkel who are battling to preserve it.

Trump's new US ambassador to Germany, Richard Grenell, infuriated the Berlin government when he said in an interview with Breitbart that his mission was to "empower" conservatives challenging the European establishment.

The administration has made overtures to Hungarian Prime Minister Viktor Orban, one of Merkel's adversaries in the EU, and Trump recently praised another populist critic of Brussels, Italian Prime Minister Giuseppe Conte.

But some senior European leaders are still trying to school Trump, even though lecturing him is often counterproductive.

"US doesn't have and won't have a better ally than EU. We spend on defense much more than Russia and as much as China," tweeted Donald Tusk, the President of the European Council, tagging the President's account.

Moments before Air Force One landed in Brussels, Tusk got an answer that could set the tone for the entire trip.

"The European Union makes it impossible for our farmers and workers and companies to do business in Europe (U.S. has a $151 Billion trade deficit), and then they want us to happily defend them through NATO, and nicely pay for it. Just doesn't work!" Trump tweeted.