Washington (CNN) "America First" is what Donald Trump promised, and it's what he's delivering, with his expected withdrawal from the Paris climate accord and his upbraiding of fellow NATO leaders in Europe last week.

A possible rejection of the global consensus on fighting climate change and Trump's view of NATO and transatlantic trade as transactional agreements that add up to a bad deal for US taxpayers, rather than guarantees of US power and global influence, will win the President kudos from his political supporters.

But the headlines in Europe following his trip -- "Boor in Chief" among them -- and first reactions to reports he may walk out of the Paris pact are the signs of a backlash against Trump's global role that are likely to have longstanding consequences, not just for America's image abroad, but for his own foreign policy aspirations.

Most seriously, diplomats, experts and former officials say European allies may be potentially less willing or less able to fulfill their decades-long role of having Washington's back in the world arena.

While in Europe, Trump lectured allies, rebuked Germany, physically shoved another leader, failed to endorse core security commitments and played coy on the Paris climate agreement while his G-7 counterparts unanimously lined up to reaffirm it.

Trump proclaimed it a victory.

"Just returned from Europe. Trip was a great success for America. Hard work but big results!" he tweeted.

Just returned from Europe. Trip was a great success for America. Hard work but big results! — Donald J. Trump (@realDonaldTrump) May 28, 2017

Seen from Europe, the performance deepened impressions formed during the presidential campaign that under Trump, the US is ceding its global leadership role in favor of an inward turn.

The president of the European Commission, Jean-Claude Juncker, offered a preview of the likely reaction from the other side of the Atlantic if Trump goes ahead and says he will leave the Paris pact during an appearance in the White House Rose Garden at 3 p.m. ET.

"I am a trans-Atlanticist. But if the American President was to say in the coming hours that he wants to exit the Paris accord, then it is our duty to say: 'You can't do that,'" Juncker said in Berlin on Wednesday night, according to the commission's office in the German capital.

In the aftermath of Trump's visit, German Chancellor Angela Merkel told her citizens on May 28 that "the times when we could completely rely on others are, to an extent, over."

Derek Chollet, a former assistant secretary of defense for international security affairs in the Obama administration, said a danger is that Europe won't want to follow Trump if he needs them.

"In his behavior, Trump is spending political capital that he might need to use if he wanted to pursue different policies or ask European leaders to make tough decisions on our behalf," Chollet said.

"Whether it's a new approach to Iran, a new approach to ISIS, he's giving them little incentive to cooperate. In fact, he's giving the opposite incentive: not to cooperate."

Trump has made pressuring Iran a central goal, floating the idea of backing out of the nuclear deal reached with the European Union and other parties. The idea that the US might walk away from the deal has frustrated Europeans after the years it took to reach the agreement.

Analysts say they're not likely to go along with the US if Trump does back out, continuing instead to allow their companies to open up to business with Tehran.

One European diplomat agreed, saying that leaders on the continent already face growing populist sentiment among citizens who are "already less inclined to back foreign interventions or move the focus from domestic issues. Trump is just going to make it harder" for those leaders to convince their publics to agree if the US asks for help, the diplomat said.

"The way the political incentives in Europe are working right now, it's better not to cooperate with the United States because Trump is so politically toxic," Chollet said.

Delivering a lasting blow

There are limits to how much both the US and Europe could disengage from one another. As the world's largest economy, the US is a huge market for European goods and the American military is crucial to European security.

Trump had said that he wants other nations to step up and do more on global challenges, including North Korea's aggressive pursuit of a nuclear weapon, which the administration wants to build an Iran-style coalition to counter.

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The administration has also pushed other countries to do more on another of Trump's foreign policy priority -- eradicating ISIS. On both fronts, Europe will be a key player.

The US also relies on European intelligence services and its military. While the the US carries the lion's share of the military load around the world, Europe has been increasingly important. In West Africa, Europe has a far more active presence than the US military, with countries like France leading the fight against terrorist groups there.

US and Germany

But Trump's own belligerent persona and sharp demands of European leaders to spend more on their militaries could be counter-productive. Germany, for instance, has committed to reaching the 2% threshold by 2024.

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The decision came after an exhaustive policy review and a conclusion that the perceived threat from Russia had grown. But the move was deeply sensitive in a nation that is still intensely conscious of the consequences of its past militarism. Now the debate on spending will be infused by hostility towards Trump, who is highly unpopular in the country.

Indeed, the animus for the President is so bad in Germany, that Der Spiegel, the country's equivalent of Time magazine, summed up Trump's visit with an op-ed that concluded: "It's time to get rid of Donald Trump."

Trump had upbraided Germany for being "bad, very bad" about its trade surpluses and berated NATO leaders about defense spending without reaffirming the core principle of mutual defense -- after telling the autocratic leader of Saudi Arabia on an earlier stop that he was not "here to lecture."

Back home, at least among Trump's allies, reviews for his trip were better.

The Republican chairman of the Senate foreign relations committee was among those who praised the performance. Sen. Bob Corker of Tennessee tweeted that he'd told Trump "that I could not be more pleased with his first international trip," adding that it "was executed to near-perfection."

Corker said he was "confident we can reassert US leadership, strengthen key alliances."

It was a stark contrast to Merkel's assessment that Europe now "really must take our fate into our own hands."

Bill Kristol, the conservative editor and commentator, said that "Merkel's comments today are a reminder that Trump's failures are, while he's president, also America's failure, and damage America."

Still, for Trump's supporters, a US withdrawal from the Paris climate accord and his earlier rejection of the Trans Pacific Partnership are a sign that the President is prepared to challenge a global establishment that they believe has hurt their economic interests to benefit elites, a form of leadership in itself.

Trump's top advisers reject the notion that his attitude is a symptom of an abrogation of US global leadership. In fact, they contend that Trump is providing direction following an era of US disengagement.

Turning its back

Twice in the first four months of the administration, Washington will have turned its back on major international agreements, which involved its partners and rival governments also making painful political concessions to reach a deal.

That experience is likely to make it less likely in future, whoever is president, that foreign states will trust the US to live up to its commitments. It will also further undermine the liberal international order and the dominant role of Western-style democracy, and cede leadership positions to American rivals like China, Trump critics argue.

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"#ParisAgreement is not only about the climate: It is also about America remaining the global leader," wrote former Republican presidential nominee Mitt Romney on Twitter on Wednesday.

Juncker made a similar point, saying he had explained to Trump that "it would not be good for the world or the United States if America was to literally step off the world's stage. Because the vacuum will be filled and the Chinese are in prime position to take on a leadership role."

Also Wednesday, Democratic Sens. Robert Menendez of New Jersey and Jack Reed of Rhode Island sent a letter to the White House saying that "leaving the Paris agreement would damage relationships with our allies and weaken American leadership on the global stage ... empowering sometimes adversarial nations like China."