BERLIN — More than 20 nations signed a letter this week forcefully condemning China’s apparent systematic detention of minority Uighurs and other Muslims — a landmark step toward holding Beijing accountable for what human rights groups and the United States have described as a policy of mass incarceration based solely on ethnicity.

The letter to the United Nations High Commissioner for Human Rights was also notable for another reason: that it was written at all.

The United States has typically taken the lead in criticism of China’s human rights record, sparing smaller nations the task of facing down Beijing’s economic and political might. But the Trump administration withdrew from United Nations Human Rights Council last June over the body’s frequent criticism of Israel’s treatment of Palestinians.

Without the United States, many expected the council to be significantly weakened. But this week’s step to condemn China has illustrated the way in which U.S. allies have begun to adapt to Trump’s abandoning of global bodies and treaties by recalibrating their own strategies. In some cases, the void left by the United States has been largely filled by other Western democracies. But in other cases, U.S. absences from treaties has allowed rivals like China or Russia to expand their influence.

The United States has rarely shied away from condemning Chinese government practices, but smaller Western democracies like Germany, France and Britain are more vulnerable to Beijing’s political and economic leverage and have often attempted a balance between private criticism and public signs of support.

But the letter issued against China this week is not attributed to any one nation and did not have a clear initiator or coordinator — a role that would previously have fallen to the United States in many instances. This time, the document was equally backed by every signatory, which makes it far more difficult for Beijing to retaliate, given that it would need to target more than 20 individual nations.

They included key American allies like Canada, Australia, France, Germany, Japan and the United Kingdom.

The move was the latest instance of an increasingly complicated diplomatic dance to accommodate Washington’s sudden withdrawal of support to important international organizations or treaties, while maintaining momentum for important global issues.

It’s proven mostly difficult.

Trump’s withdrawal from the Paris climate accord in 2017, for example, limited Europe’s ability to push ahead with tougher emission limits. Overall, the withdrawal appears to have slowed down global climate action, according to a report released by the Institute of International and European Affairs think tank at the end of last year.

In other cases, the impact of U.S. withdrawals is likely to take years to fully materialize. When the United States left the United Nations Educational, Scientific and Cultural Organization (UNESCO) earlier this year, China quickly sensed an opportunity to expand its own influence there. Beijing has pushed for aggressive development on world heritage sites.

Its expanded role within UNESCO, wrote political scientist Carsten Vala in a contribution to the Berkley Center for Religion, Peace & World Affairs at Georgetown University, may result in Beijing being more successful at promoting “alternatives to the view that heritage sites should be ‘original’ constructions and allow for their rebuilding, renovation, or expansion.”

In other instances, the Trump administration’s departure from international organizations or deals has resulted in U.S. allies and rivals suddenly finding themselves on the same side — and opposed to the United States. After Trump withdrew from the Iran nuclear deal last year, Europe, Russia and China stuck with the agreement and U.S. allies even openly plotted how to circumvent U.S. sanctions on Iran, creating the basis for a financial transactions mechanism that could also be used by U.S. adversaries in the future.

Adapting to the new realities of global diplomacy under the Trump administration has taken some time for Western diplomats. Canada had to learn this the hard way after its Foreign Minister Chrystia Freeland criticized Saudi Arabia for arresting women’s rights activists last summer. Riyadh responded by halting trade deals and commercial flights, suspending diplomatic ties and ordered Saudi students to return home.

The Trump administration refused to publicly intervene on Canada’s behalf.

Beijing has routinely responded defensively to accusations issued by the United Nations and rights groups that the country’s authorities are holding as many as 1 million Uighurs and members of other minority groups — mostly Muslim ones — in internment camps in the Xinjiang region.

On Thursday, China’s foreign ministry reacted angrily to the UNHRC move, calling this week’s letter a “flagrant interference in China’s internal affairs.”

“The Chinese government and Chinese people are best qualified to speak on Xinjiang issues, and we do not allow any other countries or powers to interfere,” ministry spokesman Geng Shuang said, adding that the Xinjiang region has not had a violent terrorist attack in more than two years due in part to the education centers.

After initially denying the existence of the indoctrination centers, China over the last year has launched an aggressive public relations counteroffensive to portray its Xinjiang program as a viable counter-extremism model.

But human rights groups applauded the letter, even though it is unlikely to have any immediate consequences. “The joint statement is important not only for Xinjiang’s population,” said John Fisher, Human Rights Watch’s Geneva director, “but for people around the world who depend on the UN’s leading rights body to hold even the most powerful countries to account.”

With or without the United States.

Gerry Shih contributed to this report from Beijing.

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