Washington (CNN) In his first year as president, Donald Trump has torn at a thread running through international affairs for 75 years -- the idea of the United States as a predictable bastion of global stability and champion of Western values.

Trump, with bull-in-a-Twitter-feed diplomacy reflecting his impulsive, combative character, has turned the United States into an unpredictable and disruptive force. His attitude is born of a belief in keeping his enemies -- and friends -- off balance and rooted in his view that the rest of the world is exploiting America and his belief that the burdens of US leadership are too costly.

Trump's smoldering rhetoric, disdain for multilateral diplomacy, distrust of trade deals and apparent racial and religious prejudices have changed the kind of behavior the world expects from a US president.

Trump maintains that his leadership is clearing up a global "mess" he inherited, and "America is being respected again abroad."

Yet his directional shift has compromised America's image abroad, left the world uncertain about US policy on key issues and has allied leaders foundering as they work out whether flattering , appeasing, teaching or ignoring Trump works best.

Acting on a mandate

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But no one should be surprised that Trump is acting on what he believes is a mandate from American voters.

A year ago in his inaugural address , Trump delivered a stark warning to a world that had watched his campaign with a mixture of fascination and horror: that long-held assumptions about US behavior were no longer operable.

"From this moment on, it's going to be America First," the President said.

The roots of his foreign policy were right there in the speech, as he painted a picture of a world that had exploited American generosity, ripped off US firms and workers and was festering with radical Islamic extremism.

Each decision flew in the face of international consensus and the best advice of foreign policy elites at home. Often, foreign policy decisions seemed to be motivated more by a desire to please the 35% or so of Americans who compose Trump's base and embrace his populist nationalism than from any sense of long-range foreign policy strategy.

Trump's fiery temperament, including flippant comments about the possibility of nuclear war, shattered conventions governing how presidents normally talk and engage with the world and have stoked fears about his readiness to lead in the event of a dangerous global crisis.

Trump's militaristic rhetoric and decision to gut his own State Department, his attacks on the press, downplaying of human rights and cozying up to autocrats raised doubts about whether he subscribed not just to Western values, but also to the fundamental principles on which America is built.

Concern, mystification

A new Gallup poll released Thursday put global approval of US leadership at just 30%, down from 48% in the last year of President Barack Obama's administration. Germany was the top-rated global power, while China had edged ahead of the United States, at 31%.

Across Europe, there is concern that issues crucial to the continent's security, like the Iran nuclear deal and climate change, are becoming collateral damage to Trump's desire to satisfy his political base.

In Asia, there is mystification about what often is viewed as a US retreat, at a moment when China is stronger and more aggressive than it has been for centuries and keen to exploit a power vacuum.

Part of the problem is that it's often difficult to work out exactly what US foreign policy is on many issues.

There is the orthodox, Republican approach pursued by Defense Secretary James Mattis and Secretary of State Rex Tillerson enshrined in the new National Security Strategy

Then there is the policy spelled out in Trump's explosive tweets that have rebuked foreign leaders, poured oil on sensitive diplomatic negotiations and kept his own subordinates in line -- as when he told Tillerson the secretary of state was "wasting his time" by trying to negotiate with North Korea over the nuclear crisis.

Yet, as always with Trump, there is a tendency for the noise and chaos he creates to trample cool analysis. Some foreign policy experts say his performance is in fact more proficient than he makes it look.

Alan Mendoza, executive director of the Henry Jackson Society think tank in London, says Trump is addressing the flaws of the Obama presidency, which he says was a period of retrenchment and decayed US prestige that left Iran rampant, Russia undeterred and North Korea racing toward a deliverable nuclear bomb and triggered human carnage in Syria.

"It is clear that President Trump is unorthodox. It is clear that he is offensive in some ways, but has he got results? In many ways he has," Mendoza said.

Mendoza said Trump's pressure appeared to have prompted European leaders to up military spending, and he praised the President for enforcing a red line against the use of chemical weapons in Syria with military action -- unlike Obama. He said Trump's approach has persuaded North Korea that the President is serious about potential military action, and may have prompted its diplomatic outreach to the South.

"There is a lot of focus on the methodology of how the Trump administration conducts its policy, and far less paid to the results of what is happening. It's only one year in but we have seen some positive signs," he added.

Power shift

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Still, there is consternation in many parts of the world about Trump's approach. In Asia, US friends worry that his withdrawal from the Trans-Pacific Partnership trade deal has ceded the primary role in writing the rules of a huge chunk of the global economy to China.

Chinese President Xi Jinping, meanwhile, has indicated he is ready to exploit perceptions that Washington is stepping back. He portrayed himself at last year's World Economic Forum as the guardian of free trade, in a groundbreaking appearance that may have prompted Trump to travel next week to this year's event in Davos, Switzerland

The power shift is worrying key US allies in the region. Singapore's Prime Minister Lee Hsieng Loong asked Americans a startling question in a speech in Washington last year: "What do you believe? Where do you stand? Do you still believe that you still have the most to gain from an interdependent world, open exchanges and multilateral rules?"

In the Middle East, the Trump administration has had perhaps its greatest foreign policy success, with the defeat of ISIS in Syria, albeit by carrying out a plan initiated by the Obama administration. Trump has repaired US relations with Saudi Arabia and is adopting a more confrontational posture toward Iran's regional ambitions than the previous White House. But his decision to move the US Embassy in Israel could have killed the Middle East peace process.

European leaders have tried various approaches to deal with Trump but have found that his behavior and rhetoric have put them in tough political positions.

British Prime Minister Theresa May put her faith in the special relationship but Trump's unpopularity, exacerbated by his retweet o f posts by a far-right British hate group, made it politically unsustainable for him to visit the UK -- a stunning development for America's President and its closest ally.

German Chancellor Angela Merkel lectured Trump on freedom, human rights and racial tolerance in her congratulatory message after his election win. After watching Trump berate fellow NATO leaders in Brussels in May, she suggested to Germans that Europe could no longer rely on the US to always be there, saying they "really must take our fate into our own hands."

Much of the world is considering similar questions after the first year of Trump's presidency, wondering whether he will succeed in a fundamental reordering of the US role in the world and what that turn inward will mean for the rest of the international system.

Allan Gyngell, national president of the Australian Institute of International Affairs, says the draining of the State Department by a secretary of state who seems uninterested in its mission adds to signs that the US is abandoning the field of international diplomacy, for example, in its withdrawals from the Paris deal and UNESCO.

"No doubt a more familiar sort of American government will take office in three or seven years, but the United States has been fundamentally altered by this experience," Gyngell said.

"The high watermark of American liberal internationalism has passed."