“Afghanistan risks becoming once again a platform for international terrorists to plan and organize attacks on our homelands,” he said. “And ISIS could rebuild in Afghanistan the terror caliphate it lost in Syria and Iraq,” Stoltenberg added, referring to the Islamic State militant group.

The warning, though couched in diplomatic language, marked the sharpest tone that Stoltenberg has taken toward President Trump. The United States is by far the most powerful member of NATO, and Trump has repeatedly clashed with the other partners and privately threatened to quit the military alliance altogether.

U.S. officials said the White House is planning to roughly halve the number of U.S. troops in Afghanistan, from about 5,000 to 2,500, by the time President-elect Joe Biden assumes office Jan. 20. The officials have said that the announcement could be made as early as this week.

A U.S. drawdown in Afghanistan would gut NATO operations in the country. Although the military alliance has about 12,000 troops from 38 countries in Afghanistan, it is reliant on U.S. personnel and infrastructure. The expectation at NATO is that if the United States pulls out, everyone else will also, given the importance of U.S. logistical capabilities in Afghanistan.

Under the leadership of the fundamentalist Taliban group, Afghanistan became a safe haven for Islamist extremists in the 1990s. After the al-Qaeda terrorist attacks on 9/11, U.S. airstrikes and Afghan resistance forces ousted the Taliban from power for hosting Osama bin Laden.

NATO forces poured in after the Taliban fell, and in 2003 a U.S.-led NATO mission took control of international security efforts in the country. In his statement Tuesday, Stoltenberg emphasized the high cost that NATO allies have paid. “Hundreds of thousands of troops from Europe and beyond have stood shoulder to shoulder with American troops in Afghanistan, and over one thousand of them have paid the ultimate price,” he said.

The U.S. election results were greeted with relief inside NATO’s glassy new headquarters in Brussels this month. Many diplomats feared that Trump, given a second term, would make good on private threats to pull the United States out of the alliance.

Stoltenberg — a former Norwegian prime minister who is light-years away from Trump in temperament — has made it his mission to be a Trump whisperer since 2017. Keep Trump happy, Stoltenberg’s advisers sometimes said, and keep NATO going.

Even in private conversation, Stoltenberg has refused to criticize the man who shoved aside Montenegro’s prime minister at a summit, routinely misrepresents NATO’s spending pledges and has appeared more eager to befriend the alliance’s main foe, Russian President Vladimir Putin, than his fellow democratically elected leaders.

Days before the 2016 election, Stoltenberg told a reporter with exasperation that Trump had exerted no influence on European defense spending, which was already rising following Russia’s 2014 annexation of Crimea.

After the election, however, NATO started boasting about increased defense spending since 2016, which was a way to write President Barack Obama out of the history and give Trump an easy public relations victory. When Stoltenberg was asked about the tactic, he would simply smile.

Tuesday’s statement appeared to be the sharpest attempt yet to push back at Trump’s go-it-alone impulses on defense issues that affect many NATO militaries. It also reflects concerns that despite Trump’s push for a peace deal with the Taliban, Afghanistan is still a risk to international security.

Trump has repeatedly threatened to pull troops out of Afghanistan, part of a broader promise to roll back open-ended U.S. involvement in foreign conflicts. His proposals have won cautious praise from critics on the left who accuse the U.S. military of ongoing overreach abroad. But foreign policy elites generally oppose rapid withdrawal from Afghanistan, fearing for the future of the country the departing troops would leave behind.

Edmund Fitton-Brown, coordinator of a United Nations monitoring team on the Islamic State, al-Qaeda and Taliban, told the BBC in October that the first two of those groups could seek to exploit a weak Afghan state. “Both of those groups have an avowed aspiration to pose an international threat,” he said.

In a report submitted to the U.N. Security Council in May, he warned that al-Qaeda maintains a presence in the country and a relationship with the Taliban, and that peace would rely on commitments by the Taliban to oppose international terrorism.

“The challenge will be to secure the counterterrorism gains to which the Taliban have committed, which will require them to suppress any international threat emanating from al-Qaeda in Afghanistan,” he wrote.