Europe’s worry: Would America care about us if Donald Trump were president?

In the United States, President Joe Biden’s political allies have spent the past week trying to calm Democratic Party anxiety over his stumbling debate performance against Donald Trump.

But another set of allies was equally shaken: America’s key overseas partners in its efforts to contain China’s expanding ambitions and to push back against Russia’s invasion and occupation of Ukraine.

For months now, concern has been growing among European member states of NATO and major allies in the Asia-Pacific region like Australia, South Korea, and Japan, over the prospect of a second Trump administration.

But in the wake of Mr. Biden’s debate ordeal, the concern has edged closer to alarm.

It is being fueled by a growing expectation that if Mr. Trump wins in November, they can no longer reliably assume that the world’s wealthiest and most powerful democracy will remain interested, involved, and invested in checking the reach of powerful autocracies.

And they’re worried that U.S. domestic political constraints mean that even if Mr. Biden wins, Washington may not be able to show anything near the interest, involvement, and investment that have shaped world politics ever since World War II.

The allies’ debate anxiety was especially evident regarding future U.S. backing for Ukraine.

Mr. Trump used the debate to reiterate his opposition to the scale of support the Biden administration has given Kyiv, and insisted that if he won the election, he’d secure a peace agreement even before taking office.

He said Russian President Vladimir Putin’s terms for a settlement were unacceptable. But his past acceptance of Russian claims on Ukraine left European leaders assuming that, at a minimum, he would recognize Russia’s control of the land it has seized by force.

While allied leaders avoided public comment on the debate, other prominent European politicians were far less reticent, with some taking the extraordinary step of calling on the Democratic Party to choose a new election standard-bearer.

Former Italian Prime Minister Matteo Renzi summed up the mood. “Joe Biden can’t do it,” he posted on X. “Changing horses is a duty for everyone.”

Poland’s foreign minister, Radosław Sikorski, reached back into history. Noting that a failure to “grasp the importance of succession” had begun the decline of the Roman Empire, he added that “it’s important to manage one’s ride into the sunset.”

The chair of the German parliament’s defense committee was blunter. “The fact that a man like Trump could become president again because the Democrats are unable to put up a strong candidate against him would be a historic tragedy that the whole world would feel,” said Marie-Agnes Strack-Zimmermann, whose Free Democratic Party is a member of the country’s ruling coalition.

A longtime former chair of the German parliament’s foreign affairs committee also homed in on the international implications. “This night will not be forgotten,” Norbert Röttgen said. “The Democrats have to rethink their choices now. And Germany must prepare at full speed for an uncertain future.

“If we don’t take responsibility for European security now, no one will.”

That, of course, is exactly what Mr. Trump has been demanding of U.S. allies in Europe for a long time.

It may be that Europe’s growing investment in defense over the past few years would dissuade a reelected President Trump from withdrawing from U.S. alliances altogether, as he suggested he might in his first term.

It may also be that the prospect of his possible return was one of the reasons behind the higher European defense spending.

Yet the main catalysts have been Russia and China, and the real sense of threat that their democratic neighbors feel in the light of the Ukraine invasion and China’s expanding military presence in the South China Sea.

President Biden and U.S. allies have made efforts to “Trump-proof” more robust security relationships.

In Asia-Pacific, Mr. Biden has put together a three-way partnership with Australia and Britain to provide nuclear submarines for the Australians. He has beefed up the Quadrilateral Security Dialogue among the U.S., Australia, India, and Japan.

And at Camp David last summer, he put America’s arms around an historic rapprochement between the leaders of South Korea and Japan – pointedly including tripartite consultation mechanisms intended to outlast his time in the White House.

A similar effort has been underway with Ukraine. The most recent example: At its summit last month, the G7 leading industrialized nations agreed to pay the interest on a $50 billion loan it will take out on Ukraine’s behalf.

Still, the allies recognize there are stark limits, at least in the short term, to how effectively they can compensate for any major reduction in America’s international presence and commitment.

They are also worried that the retreat is already underway, at least on Ukraine; President Biden struggled for months last year to win congressional approval for America’s most recent aid package.

One leading Japanese foreign policy expert, former diplomat Kunihiko Miyake, lamented after the debate that Mr. Biden “might have imploded,” suggesting that Mr. Trump could have a clear run at the White House.

“Unlike eight years ago,” he said, “we are much more prepared, as are other European and Asian allies.”

But he added a caveat. “Still, Mr. Trump is unpredictable.”