President Donald Trump's recent interactions with Europe on climate change, mutual defense and military spending are chilling decades-old relationships with America's closest allies and leaving world leaders and observers alike to wonder whether the world could be witnessing the end of the American century.

Born from the unprecedented American investment in rebuilding the continent after World War II, these relationships were anchored by the U.S. commitment to institutions like NATO and the European Union and broadcast to countries worldwide the benefits of alliance with the West.

But Trump's actions during his recent overseas trip followed on statements indicating he wishes for traditional partners to do more for themselves and, taken together, have been interpreted in Europe and elsewhere as a tacit withdrawal from America's role as an international benefactor, advocate and enforcer.

"I think it is fair to say that trans-Atlantic relations are at their absolute low point over the course of the four decades of my career," says Jim Stavridis, the former Navy admiral and NATO supreme allied commander Europe who now serves as dean of the Tufts University Fletcher School of Law and Diplomacy. "We need to wake up and realize that our best pool of partners lies directly across the Atlantic and find ways to cooperate and ensure mutual respect."

The president's reported inclination to withdraw from the Paris Agreement , which former President Barack Obama's government entered in September , puts the U.S. among a handful of countries – including Angola, Iran and Russia – that have not ratified an initiative the former U.N. secretary general stressed as " urgent ." It signals, among his other actions, "that President Trump does not value international order," Richard Haass, the president of the Council on Foreign Relations, tweeted .

Trump's trip left German Chancellor Angela Merkel openly speculating about a future for Europe without the same U.S. leadership – a stinging critique from a seasoned stateswoman increasingly seen as the de facto leader of Europe itself.

"The times when Europe could rely solely on others is somewhat in the past," she said at a political rally on Sunday. "And as I have witnessed over the past few days, Europe must take its fate into its own hands. This means working in friendship with the U.S., the U.K., and neighborly relations with Russia and other partners."

White House spokesman Sean Spicer, reading the translation of Merkel's comments aloud at a press briefing Tuesday, said the chancellor's response was "great."

"That's what the president called for," Spicer said.

Top White House officials, including national security adviser H.R. McMaster, have claimed Trump accomplished precisely what he set out to do in his journey through Saudi Arabia, Israel, and to Europe for NATO and G-7 meetings.

"America First does not mean America alone," the Army lieutenant general wrote in a Tuesday op-ed in The Wall Street Journal. He described Trump's commitment to protecting America's vital interests and demanding other countries to contribute more, which McMaster said "deepens our friends' respect for America."

"By asking for more buy-in, we have deepened our relationships," he wrote.

Steven Pifer, a career U.S. diplomat who served in Poland, the U.K. and Russia before becoming ambassador to Ukraine, believes the world looks to America's commitment to NATO countries as the bellwether of its foreign standing.

"There is a perception out there now … that U.S. leadership of NATO is weakening, that we're drawing back from that," says Pifer, now a senior fellow at the Brookings Institution. "That may raise questions about our ability to be a player and to lead on other questions."

Pifer points to what he considers a juxtaposition between Trump's apparent comfort in visiting Saudi Arabia without highlighting its questionable human rights record before lecturing NATO leaders a few days later about spending more on defense. Those remarks took place in Brussels during the unveiling of a monument to the alliance's collective defense agreement, known as Article 5.

"It is doing damage," Pifer says of Trump's actions. "The trans-Atlantic relationship may be more resilient than one president, but it seems to me that there are a lot of big issues out there that it is in the U.S. advantage to have partners in dealing with these. In the extent that we're seen as drawing back, or weakening the trans-Atlantic ties, we may well find that we have fewer partners when we want to bring a group together and do things."

Some fear that Russia might test U.S. plans to stand by its previously unshakable commitment to its allies' defense and implement the kind of hybrid warfare it previously deployed to Ukraine to other former Soviet states, like the Baltic countries that are now members of NATO.

And some believe Trump's behavior toward NATO countries could provide precisely what the alliance needs.

In an op-ed written in advance of the president's trip, former NATO Secretary General Anders Fogh Rasmussen claimed Trump's foreign policy rhetoric has "triggered a healthy process, prompting European allies to ask, not if the transatlantic alliance can survive, but how they can ensure its longevity and vitality?"

Rasmussen cited Trump's military activities in his first months in office, including targeting a Syrian air base, dropping the U.S. military's largest non-nuclear bomb on an Islamic State group complex in Afghanistan and requesting the Chinese do more to confront North Korea, in defending the former business mogul and reality TV star's transactional approach to interactions with foreign powers. He called on Trump to reaffirm America's support for Article 5 – which the president did not do explicitly in his remarks, though he promised to "never forsake the friends that stood by our side" – and for NATO partners to focus on spending the collective goal of 2 percent of their gross domestic product on defense.