(CNN) Foreign policy, increasingly, is what is happening around the world while the United States is making other plans.

More than five months into Donald Trump's presidency, American adversaries and allies alike are adjusting to a new era in which Washington seeks its own idiosyncratic and unpredictable "America First" path.

In Europe, Asia, and the Middle East, governments are assessing shifting US priorities and in some cases seeking alternative sources of leadership and partnership in the belief that America has stepped back.

Trump's unpopularity abroad is forcing leaders to consider their own political positions, before getting too close to the American President -- even if they seek to preserve Washington's still vital global role as the guarantor of liberal market economics and democracy.

That dynamic will be on display during Trump's second visit to Europe this week, just weeks after his first transcontinental trip opened new gaps between Washington and some longtime allies.

Trump starts in Poland, which is hoping for his strongest affirmation yet of NATO security guarantees. Then he will head to the G20 summit in Germany, where he may confront hostility deepened by his decision to exit the Paris climate accord.

The Trump administration refutes the notion that it has downgraded American leadership, arguing that Trump's foreign trips, flurry of meetings and frequent calls with foreign presidents and prime ministers shows intense engagement.

But increasingly, top foreign policymakers from Germany to Iraq and Canada to Asia are contemplating a period when US leadership that many took for granted may be less evident in global affairs, after Trump turned his back on multilateral trade deals and downplayed multinational institutions and agreements.

"Whoever believes the problems of this world can be solved by isolationism and protectionism is making a tremendous error," German Chancellor Angela Merkel told parliament last week, in a clear shot across Trump's bow.

It was not the first time the German leader, running for a fourth term in September's election, had rebuked the President.

After Trump visited Europe in May, and declined to reaffirm NATO's Article 5 principle of mutual self defense during a visit to the Western alliance headquarters, Merkel said US allies needed to rethink their place in the world.

"We Europeans truly have to take our fate into our own hands," she said.

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Canada, America's closest geographical ally, is also watching.

Foreign Minister Chrystia Freeland profoundly thanked the United States for being "truly the indispensable nation" that had ensured 70 years of peace and prosperity in a speech to parliament last month.

But she acknowledged that halcyon period was ending.

"The fact that our friend and ally has come to question the very worth of its mantle of global leadership, puts into sharper focus the need for the rest of us to set our own clear and sovereign course," Freeland said.

"For Canada that course must be the renewal, indeed the strengthening, of the postwar multilateral order."

It is not just America's most traditional allies that sense that America is pulling back from the world, amid a perception that diplomacy has been de-emphasized and the State Department downgraded in a Trump administration more respectful of military leadership.

Iraqi Vice President Ayad Allawi told CNN's Christiane Amanpour last week that the United States was "absent" in maintaining global security and that there was a "vacuum in the overall leadership in the world."

"The Americans need to ... get back to their role as an international power, an important international power." Allawi said.

Despite an impending victory over ISIS by Iraqi forces in western Mosul, with US support, Allawi argued that Washington lacked "clear cut policies" for tackling extremism and a future strategy for the Middle East.

Some American competitors see an opening.

At the Global Economic Forum in Davos, a few days before Trump was inaugurated, China's President Xi Jinping, offered a vision of a world turned on its head when he offered his own nation as a guardian of free trade, globalization and efforts to combat climate change -- areas where the United States had formerly taken the leadership role.

"Whether you like it or not, the global economy is the big ocean you cannot escape from," Xi told delegates at the Swiss mountain resort.

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'America First' or 'America alone?'

Over the last few days, Trump has spoken to leaders of US allies in the Gulf, amid a showdown over terrorist financing that has led to the isolation of Qatar, and has also had conversations with counterparts in Germany and Italy.

In contrast to the way Trump's first trip to Europe was seen across the Atlantic, national security adviser H.R. McMaster argued that the President had reinvigorated US alliances which Republicans believed eroded under the Obama administration.

"America First ... does not mean America alone. President Trump has demonstrated a commitment to American alliances because strong alliances further American security and American interests," McMaster told reporters last week.

While much of America's future foreign policy course remains uncertain to foreign states, Washington has made some clear moves.

It significantly stiffened resistance to Iran in the Middle East, a reorientation that was the underlying theme of Trump's first stops in Saudi Arabia and Israel.

But at the same time, there is no real clarity on the Trump administration's strategy on Syria following the apparently imminent eradication of ISIS strongholds. Iran envisages a future Shiite crescent of influence, that would stretch from Tehran through Iraq, Syria and into Lebanon, backed by Russia, and would change the balance of power in the region.

It is unclear how actively the Trump administration plans to resist such a scenario, in concert with allies like Saudi Arabia, the Gulf States, Egypt and Jordan.

In Afghanistan, the Pentagon dropped its largest non-nuclear bomb on ISIS targets and plans to use its new autonomy under Trump to send more troops to train and assist Afghan soldiers.

But the administration has yet to lay out a detailed vision of how it sees Afghanistan's future or long-term US war aims.

In Asia, Trump dropped his hostility toward China in an effort to convince Beijing to do more to rein in its volatile ally North Korea amid a nuclear and missile crisis. But he now seems to have concluded the effort failed, and imposed sanctions against a Chinese bank with links to the pariah state, and approved a $1.4 billion arms package to Taiwan, heightening tensions with Beijing.

But Trump, despite saber rattling, has yet to explain to Americans any new approaches on how he will thwart Pyongyang's bid to put a nuclear warhead onto a weapon that could reach the US mainland.

It's not just uncertainty about American global strategy that is convincing some allied leaders to look past the United States.

Trump's unpopularity makes it much more difficult for them politically to support him. The recent Pew Global Attitudes poll showed Trump with rock bottom approval ratings across the world. Only in Russia and Israel did more people trust him to do the right thing than former President Barack Obama.

The former President, meanwhile, has stayed mostly out of the limelight. But Monday, Obama couldn't resist during a Seoul conference organized by South Korea's Chosun Ilbo media group, saying the Paris climate accord won't vanish despite the "temporary absence" of American leadership.