President Trump's decision to pull the United States out of the Paris climate agreement this week sent shockwaves through global diplomacy.

Half a dozen world leaders lambasted the Thursday announcement, which Trump said was meant to protect American businesses from the “unfair” terms of the climate accord negotiated in 2015.

Former diplomats and experts warned that the U.S.'s disengagement on climate could be a defining moment for a Trump administration that is focused more on an "America First" agenda than engaging with international allies.

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“I really think that is the major consequence of today: it’s not about the Paris agreement,” said Christiana Figueres, the former head of the United Nation’s climate change mission.

“I think the real problem today and the real sadness is the absolute death blow to the international credibly of the current U.S. leadership. … The blow to the international political credibility of the United States really cannot be underestimated.”

Trump’s Paris decision capped a stretch of foreign encounters that caused allies to publicly raise concerns about their relationship with the United States.

His announcement came one week after a particularly tense trip to Europe that left key leaders at a loss on how to deal with Trump, who won last year’s election riding a wave of populist and nationalist sentiment.

Even before the Paris decision, U.S. allies were concerned about a host of Trump administration policies covering issues like trade, international security and immigration.

During a speech in Europe, Trump did not mention support for the Article 5 component of the NATO treaty, which stipulates an attack on one country is an attack on the whole alliance. He has repeatedly said other countries need to increase their defense spending to keep NATO viable.

Meanwhile, a Group of Seven (G7) summit turned into a debate on trade policy, an area where Trump feels the U.S. is short-changed. German Chancellor Angela Merkel later said the meeting included a "very intense exchange" on climate change in which all six other leaders urged Trump stay in the deal.

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Merkel — perhaps Europe’s most powerful leader and a close ally of former President Obama — appeared frustrated with Trump after the summit, saying Germany could no longer “completely depend” on the United States and that European leaders “must really take our destiny into our own hands.”

Trump’s relationship with other leaders is similarly frayed. His first impression of new, centrist French President Emmanuel Macron were dampened by a hostile handshake from the Frenchman, the Washington Post reported on Thursday.

Macron responded to Trump’s climate decision with a strong statement — delivered live at 10 p.m. in Paris on Thursday night — and sent a mocking tweet that read, “Make Our Planet Great Again.”

France, Italy and Germany released a joint statement on Thursday night saying the Paris deal could not be negotiated, despite Trump’s talk of getting a better deal.

Other reactions were more muted. Canadian Prime Minister Justin Trudeau — who has already clashed with Trump on trade policies — tweeted that he was “disappointed” in the U.S.’s decision, but committed his country to climate action.

British Prime Minister Theresa May released a similar statement, though the UK, dealing with its own surge of nationalist fervor after Brexit, avoided denunciations of Trump.

But other allies did not hold their fire. Belgian Prime Minister Charles Michel, for instance, called Trump’s decision a “brutal act.”

The White House has dismissed concerns that the Paris withdrawal will erode America’s influence over international affairs.

EPA Administrator Scott Pruitt told reporters on Friday that he expects other nations will maintain their desire to work with the U.S. on energy technology development despite the Paris decision.

National Economic Council Director Gary Cohn, a Paris deal supporter, told CNBC on Friday that the U.S. will always “have a very important seat at the table” in international discussions.

“We're not America alone, we're part of a world, an important part of the world. World leaders look to us,” he said.

“The other world leaders are always involved with us and we're involved with them, and we're going to continue to have that position in the world. We're going to the G20 in another four weeks. And the posture that we're going to have there is the same posture we’ve always had.”

But diplomats say Trump’s decision— combined with his aggressive posture on trade and security issues — has already damaged the U.S.’s standing with key allies.

“I think something very, very serious is going on right now, and the question will be whether the next U.S. president can rescue this, quite frankly,” said Andrew Steer, the president of the World Resources Institute and a former World Bank official.

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“What’s going on today is profoundly important in terms of geopolitics.”

Steven Pifer, a Brookings Institution scholar and former State Department official, said a weakened U.S. serves only empowers Russia, which has long looked to diminish U.S. influence around the world.

“My guess is, in the Kremlin, they are absolutely delighted,” Pifer, a former ambassador to Ukraine, said.

“To the extent that withdrawing from the Paris accord looks like the United States stepping back from an area where the U.S. has been leader, that’s simply music to [Vladimir] Putin’s ears.”

Diplomats said Trump’s tone could exacerbate his problems in dealing with other countries.

Figueres, who was instrumental in crafting the Paris accord, said Trump’s defiant, 20-minute Rose Garden speech against the deal would play badly with its strongest international supporters.

Similarly, Pifer said Trump’s decision to lecture European leaders about NATO during an event at the Article 5 memorial was likely taken poorly by allies. Article 5, he noted, has only been invoked once — after the 9/11 terror attacks on the United States.

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“The diplomacy that’s being pursued by this administration — it’s strange to figure out and it’s going to be ultimately self-defeating,” he said.

Domestically, Democrats and former Obama administration diplomats were aghast at Trump’s Paris decision.

Leading congressional liberals slammed the deal, with Sens. Bernie Sanders Bernie SandersThe Hill's Campaign Report: Trump faces backlash after not committing to peaceful transition of power Bernie Sanders: 'This is an election between Donald Trump and democracy' The Hill's 12:30 Report: Trump stokes fears over November election outcome MORE (I-Vt.) and Elizabeth Warren Elizabeth WarrenHillicon Valley: Subpoenas for Facebook, Google and Twitter on the cards | Wray rebuffs mail-in voting conspiracies | Reps. raise mass surveillance concerns On The Money: Anxious Democrats push for vote on COVID-19 aid | Pelosi, Mnuchin ready to restart talks | Weekly jobless claims increase | Senate treads close to shutdown deadline Democratic senators ask inspector general to investigate IRS use of location tracking service MORE (D-Mass.) both calling the decision an “abdication of American leadership.”

Todd Stern, Obama’s climate envoy to the UN, wrote in an op-ed for The Atlantic that, “pulling out of Paris would cause serious diplomatic damage” and “would be read as a kind of ‘drop dead’ to the rest of the world.”

Obama’s UN Ambassador Samantha Power Samantha Jane PowerSupport swells for renaming Edmund Pettus Bridge in Selma to honor John Lewis after his death 'Obamagate' backfires: Documents show Biden, Obama acted properly 'Unmaskings' may be common — and that's the problem MORE tweeted simply, “The end of the American Century.”