“Mattis was the last man standing for what had been U.S. foreign policy since World War II,” said Norbert Röttgen, chairman of the foreign affairs committee in the German parliament and an ally of Chancellor Angela Merkel. “With him gone, this really marks a juncture in the Trump presidency. Now we have an unrestrained Trump, which is a dangerous signal for the year ahead.”

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In China and Russia — U.S. adversaries that were cited in Mattis’s resignation letter as deserving of tough treatment — there was open anxiety that the world had just become more vulnerable to conflict.

“Our concern is who comes next,” said Yue Gang, a retired People’s Liberation Army colonel and military commentator in Beijing. “If Trump chooses a lackey who isn’t willing to serve as a balance to his instincts, the worry is that the world becomes even more unstable.”

The announcement Thursday that Mattis would step down came after a dispute with Trump over the president’s decision to withdraw troops from Syria — a choice that allies said they had not been consulted on. The secretary’s departure was followed by word that the president was also preparing to downsize the U.S. military presence in Afghanistan.

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In his resignation letter, Mattis implicitly rebuked the president for undermining U.S. alliances and failing to recognize the threat posed by the United States' enemies.

Trump has rattled global confidence many times before: by questioning the U.S. commitment to defend NATO allies, by cozying up to Russian President Vladimir Putin and by jousting with America’s democratic partners at every turn, while seeming to give authoritarians a pass.

But in regions of the world that bear the imprint of decades-long U.S. influence, the Mattis news crystallized some of the worst fears about the trajectory of Trump’s foreign policy: an increasingly volatile, unreliable and inward-looking United States.

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“A morning of alarm in Europe” was how Carl Bildt, co-chairman of the European Council on Foreign Relations and formerly prime minister of Sweden, described the reaction to news of the defense secretary’s exit.

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Mattis, he wrote on Twitter, “is the remaining strong bond across the Atlantic in the Trump administration. All the others are fragile at best or broken at worst.”

Röttgen said Mattis’s exit and the U.S. withdrawal from Syria and Afghanistan should prompt Europe to step up and fill the void left behind.

“We have to be mature, and we have to rely more on ourselves,” said Röttgen, whose country has about 1,000 troops in Afghanistan and none on the ground in Syria. “How many more wake-up calls do we need in order to engage?”

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The concern felt in Berlin was no less pronounced in Paris, where François Heisbourg, a former French diplomatic adviser, wrote on Twitter that Mattis had stabilized a dysfunctional administration and “helped preserve the Western alliance system.”

Without him, he said in an interview, Europe could no longer rely on that system with any confidence.

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“The pressure [on Europe] to do something radical is going to increase,” said Heisbourg, who is president of the International Institute for Strategic Studies. “It won’t simply be about hedging. It will actually be about preparing Plan B rather than assuming we’re going to stick with Plan A.”

The French government committed to keeping troops on the ground in Syria even without U.S. involvement. Defense Minister Florence Parly said Friday on RTL radio that the Islamic State has been reduced but not eliminated. She said the French government “does not at all share” Trump’s interpretation that the fight in Syria is over.

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Parly also saluted Mattis as a “partner” and a “great soldier.”

Britain, the United States’ most active partner in Syria, appeared caught off guard by Trump’s decision to withdraw troops. Karin von Hippel​, director general of RUSI, a London-based think tank, said Mattis had “done the best job possible of controlling Trump’s isolationist impulses.”

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But with Mattis gone, countries such as Britain will be left “trying to figure out how to continue to push these liberal values that we have believed in and promoted since the Second World War without America’s leadership.”

Even countries that don’t share those values were on edge Friday.

“Mattis was tough, but not without realism: He didn’t seek conflict with Russia,” lawmaker Alexey Pushkov, a foreign-policy specialist in the upper house of Parliament, wrote on Twitter. “Will the replacement be for the better?”

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But another lawmaker in the upper house, Foreign Affairs Committee Chairman Konstantin Kosachev, saw a heartening sign in the general’s departure. On issues such as relations with Russia and China, Trump’s view was so different from his defense secretary’s that the president essentially forced Mattis out, Kosachev said in a Facebook post.

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“This is an interesting signal — likely a positive one,” Kosachev said.

As for the Kremlin’s official response, spokesman Dmitry Peskov, asked about Mattis as a restraining influence on Trump, had this dry rejoinder: “In our times, guessing who restrained President Trump from doing what is the work of political scientists and a rather thankless task."

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Mattis has persistently warned about China as the greatest long-term threat facing the United States, a view that permeated last year’s National Security Strategy paper, in which the administration recast China as a competitor. While Beijing saw Mattis as one of its toughest critics, it also saw him as a straight-shooter in an administration that the Chinese government otherwise has struggled to decipher.

Yue, the retired People’s Liberation Army colonel, said the Chinese military had high regard for Mattis, who, even in times of high tension, stressed the importance of avoiding a shooting war between the two powers.

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“Even though toward us he was tough and vexing, the Chinese military felt assured dealing with this type of professional military man,” Yue said.

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It is largely because of Mattis, “viewed as a ‘mature guy’ within the Trump administration,” said Shen Yamei, a researcher at the China Institute of International Studies, that “military relations between China and the U.S. have been relatively stable.”

In parts of South Asia and the Middle East, warnings emerged that the abrupt shift in strategy propelling Mattis’s exit would be a grievous mistake, even as Kabul struck a calm note.

A senior official in Pakistan’s Foreign Ministry, speaking about the possibility that Trump might remove about 7,000 of the 14,000 U.S. troops deployed to Afghanistan, said that “any troop withdrawal or major reduction in their number before peace is restored would be a very unwise move. It would bring chaos and disorder, more fighting and perhaps a civil war.” The official spoke on the condition of anonymity because he was not authorized to comment on the matter.

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But in the first public comments from the Kabul government on reports of the U.S. troop reduction, several aides to President Ashraf Ghani sought to put a brave face on the development, saying that such a drawdown would have no major impact on Afghanistan’s ability to defend itself. Their reaction marked a notable contrast with prior appeals for U.S. troops to remain as a force for stability.

Israel, meanwhile, was most alarmed by Trump’s abrupt decision to abandon Syria.

There was a sense that, with Mattis on the way out, Israel was losing a voice for its security interests in the Middle East. Michael Oren, a deputy in the prime minister’s office and former Israeli ambassador to the United States, noted that, like Israel’s leadership, “Mattis believed that a strong American presence in the Middle East served as a buffer to Iran and other hostile elements.”

“Today as in the past, Israel will have to defend itself with its own forces to deal with the great threats in the north,” Oren tweeted.

News of Mattis’s departure reverberated on the Korean Peninsula, where Pyongyang has said this week it will not give up its nuclear arms until the “U.S. nuclear threat to Korea” is eliminated.

Kwon Bo-ram, a researcher at South Korea’s state-run Institute for Defense Analyses, said the uncertainty created by the resignation could affect the ongoing defense cost-sharing talks between Seoul and Washington. The two sides failed to reach a deal amid disagreement over a bigger South Korean share of the cost.

In Iraq, Mattis has loomed large -- first as a Marine commander during the bloodiest battles in Fallujah following the American invasion in 2003 and later as a sympathetic ear to Iraqi officials.

Former officials expressed unease at the prospect of a Pentagon without Mattis’s intimate knowledge of Iraq’s precarious security environment.

“The task of getting through to the Trump administration has become much harder," said an adviser to former Iraqi Prime Minister Haider al-Abadi who spoke on the condition of anonymity, “if not impossible.”