America is back. But for how long, the world wonders.

President Joe Biden’s first 100 days is a landmark that’s drawing attention abroad, as well as at home. And the signs are that friend and foe alike have concluded that he is delivering what he pledged when he took office: reengaging America as a leading force on the international stage.

Yet there’s an elephant in the room. Quite literally, because the elephant in question is the symbol of America’s Republican Party – still in the grip of former President Donald Trump, who took a very different view of foreign affairs. With midterm congressional elections just 18 months away, and another presidential vote two years later, both allies and rivals are keenly aware that America’s new engagement and leadership may not be set in stone.

The seismic shift Mr. Biden has already effected was vividly on show last week when he hosted some 40 world leaders at a virtual summit to reinvigorate the 2015 Paris Agreement on combating climate change. Mr. Trump dismissed human-made global warming as a hoax, and pulled the U.S. out of the Paris accord.

Allies were especially effusive in welcoming the new president’s approach. German Chancellor Angela Merkel professed herself “delighted to see that the United States is back to work together with us.” Italian Prime Minister Mario Draghi declared, “It’s a complete change. Now we are confident that together we will win this challenge.”

Even potential holdouts were supportive. Brazil’s President Jair Bolsonaro, a fulsome admirer of Mr. Trump, praised President Biden for convening the meeting. In the wake of considerable pre-summit diplomacy by U.S. officials, he announced a new commitment to rein in deforestation in the Amazon.

Equally striking were remarks from the leaders of America’s main rivals, China and Russia. Neither Chinese leader Xi Jinping nor Russian President Vladimir Putin touched on the major areas of tension with the Biden administration, which had some pundits predicting they might not show up at all. Instead, they pledged support for the international cooperation on climate change that Mr. Biden was hoping to galvanize.

The only sting in the tail? Mr. Xi’s pointed allusion to Mr. Trump’s withdrawal from the Paris Agreement. Cooperation on climate change, he said, hinged on the assumption that “we must honor commitments, not go back on promises.”

The question that comment raised – the “elephant in the room” question on everyone’s minds – was whether a future American administration might not reverse course again.

Because the lesson other countries have drawn from the past two U.S. administrations is that America’s appetite for leadership, even for sustained involvement in the wider world, may ultimately hinge on the course of domestic U.S. politics.

To some degree, that’s always been true. “America first” isolationism has ebbed and flowed for more than a century: dogging President Woodrow Wilson’s failed effort to champion the League of Nations after World War I, challenging Franklin D. Roosevelt’s support for America’s entry into World War II, and questioning the role of America as the nascent superpower after that war.

Nevertheless, U.S. engagement and leadership enjoyed bipartisan backing throughout the Cold War, in a consensus that generally held until the turn of the 21st century.

What particularly unsettled U.S. allies about Mr. Trump’s administration was its unprecedented lurch away from foreign-policy-as-usual. It wasn’t so much any one particular policy change as the active denigration of long-standing partnerships and withdrawals from international agreements in which Washington had played a leading role.

Can foreign policy bipartisanship be rekindled? Perhaps. That’s actually been happening on two top-priority foreign policy issues: U.S. relations with China and Russia. Yet in areas like climate change, not so much, at least not yet.

The deeper challenge for Biden’s next 100 days, and beyond, is whether U.S. foreign policy can be insulated from bitter partisan battles over domestic politics.

In other words, whether Mr. Biden succeeds in another of his inaugural priorities: healing divisions at home and bringing the country closer together.

In the meantime, on the world stage, he is steaming ahead, cementing his commitment to active international reengagement. He’ll be going to Britain in June for the summit of the Group of Seven, made up of economically developed countries. Then he’ll head to Brussels to meet NATO leaders. The U.S. is also emerging as a key player in preparations for the Paris follow-up conference on climate change to be hosted by British Prime Minister Boris Johnson later this year.

He is leaving rivals like China and Russia, and less reliable allies such as Brazil and Turkey, in no doubt as to Washington’s new international resolve. Mr. Bolsonaro experienced that firsthand, in the form of U.S. pressure before last week’s climate summit to deliver a tangible commitment to tackle a problem he has long played down.

A few days ago, Mr. Biden sent a similar political message to Turkey, which has angered the U.S. and other NATO partners by purchasing anti-aircraft batteries from Russia. The U.S. president broke a decadeslong taboo, and ignored Turkish political sensitivities, when he explicitly referred to the Turks’ early-20th-century massacre of Armenians as genocide.

That comment cannot be unsaid, no matter what follows the Biden administration. But U.S. allies are waiting for more than that. Only when they see Republicans and Democrats agree on America’s role in the world will they be reassured.