The Europeans were unnerved. The Russians skeptical. The Chinese stayed silent.

The revelation that after two years in the job, Defense Secretary Jim Mattis was resigning citing policy differences with President Donald Trump led to dismay on Friday but also a period of reflection among longstanding American allies and adversaries.

"A morning of alarm in Europe," former Swedish prime minister and now chairman of the European Council on Foreign Relations, a think tank, Carl Bildt wrote on Twitter.

"SecDef Mattis is the remaining strong bond across the Atlantic in the Trump administration. All the others are fragile at best or broken at worst," he added.

François Heisbourg, previously a senior adviser to the French government and now chairman of the Geneva Center for Security Policy, a research foundation, took to the social media platform to say: "This is big bad." Heisbourg noted that the "issue is not whether Jim Mattis was a stellar SecDef but whether he 1) exercised a stabilizing influence in a dis-functional administration & 2) helped preserve the Western alliance system. Believe me, America's allies are already reviewing all options."

Mattis' announcement that he would depart an administration that has seen a flood of resignations, firings and departures took Washington by surprise. In the past three months alone, Secretary of the Interior Ryan Zinke, White House chief of staff John Kelly, Attorney General Jeff Sessions and U.S. Ambassador to the UN Nikki Haley have all chosen to leave or been pushed out of Trump's inner circle. Trump is on his second secretary of state. Rex Tillerson was fired in March, replaced by Mike Pompeo.

More:Read Defense Secretary Jim Mattis' resignation letter

Mattis detailed his decision to quit in a letter that, while it did not mention Trump by name, came after the president said he would pull U.S. forces out of Syria, where they have been helping battle the Islamic State group. Mattis opposed that idea and also a subsequent order from Trump to start a further troop drawdown in Afghanistan. The outgoing defense secretary is the architect of a policy in Kabul that has sought to stay engaged in a nation with deep security and governance flaws.

Turkey's President Recep Tayyip Erdogan said Friday that his country would hold off on a planned offensive in northeastern Syria against Kurdish forces because of Trump's planned troop withdrawal, which it welcomed. Turkey treats Syrian Kurds as terrorists. Kurdish-led fighters are also fighting the Islamic State group.

"My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held and informed by over four decades of immersion in these issues," Mattis wrote. "We must do everything possible to advance an international order that is most conducive to our security, prosperity and values, and we are strengthened in this effort by the solidarity of our alliances."

"Now the question is who will replace Mattis?" Russian lawmaker and chairman of a parliamentary foreign affairs committee Alexey Pushkov wondered on Twitter. "The replacement of Tillerson with Pompeo only intensified anti-Russian behavior in U.S. policy and reduced communication to almost zero," he said. Pushko added that Mattis was a "tough" but "not without realism ... Will the replacement be any better?"

Russian President Vladimir Putin's spokesman Dmitry Peskov said when "stability and predictability give way to unpredictability" it's a "cause for discomfort and concern."

In Australia, politician and ex-general Jim Molan told that country's media that in military and foreign policy matters Mattis was "referred to as one of the adults in the Trump administration." He said that his departure, set for Feb. 28, was highly worrying for U.S. allies in that it meant there would be "another extreme variable" in how the White House reaches foreign policy decisions that have impact on its global partners.

"I'm worried this has to do with plans for Iran, where the State Department has said we are accumulating 'risk of escalation,'" Ankit Panda, a security and defense analyst, said on Twitter, referring to long-running speculation that Pompeo, National Security Adviser John Bolton and other Iran hawks close to Trump may be seeking to push the president toward an open conflict with Tehran after the U.S. withdrew from the 2015 international nuclear accord and re-imposed sanctions on the Middle East nation.

There was no immediate reaction to Mattis' abrupt resignation from South Korea, which is at a critical juncture and relying on the U.S. as the two countries navigate fragile nuclear talks with North Korea. Mattis has also been a vocal advocate of maintaining the 28,000 U.S. troops who are stationed on the Korean Peninsula.

Trump has questioned whether it’s worth it and balked at the expense.

Japan’s secretary of defense, Takeshi Iwaya, told the New York Times that the resignation took him by surprise. "Not only did he work for close cooperation between Japan and the U.S., he also showed very strong leadership. I am hoping that the policy of close cooperation as allies will continue," he told the paper.

There was no official reaction from China, a country Mattis has described as a "strategic competitor" and criticized for its activities in the South China Sea that include sending its warships into waters and territory claimed by other nations.

However, Trump offered up his own assessment.

"There has never been a president who has been tougher (but fair) on China or Russia - Never, just look at the facts. The Fake News tries so hard to paint the opposite picture," Trump tweeted Friday, an apparent reference to Mattis' impending exit.

More:Defense Secretary Jim Mattis leaving his job after clash with Trump over Syria

More:Read Defense Secretary Jim Mattis' resignation letter

Contributing: Deirdre Shesgreen