In this Nov. 9, 2018, file photo, Defense Secretary Jim Mattis waits outside the Pentagon. President Donald Trump says Mattis will be retiring at the end of February 2019 and that a new secretary will be named shortly. Credit: Pablo Martinez Monsivais | AP

Turnover in the Trump administration is so frequent that some departures barely generate headlines. But the resignation of Defense Secretary James Mattis is cause for genuine alarm for many reasons.

Mattis was one of a few seasoned, level-headed Trump advisors who were considered the “ adults in the room.” These men were believed to stop or at least ameliorate some of the president’s worst impulses.

With the departure of Mattis, scheduled for late February, all of these “adults” will have left the administration.

Trump’s chief of staff John Kelly was essentially fired earlier this month. National Security Advisor H.R. McMaster left in April. All three men were generals with decades of experience at the highest levels of the military and government. They were thought to bring a bit of stability and discipline to a predictably unpredictable White House.

With their departures, and that of Secretary of State Rex Tillerson earlier this year, neo-conservative, isolationist, anti-immigration hawks such as national security adviser John Bolton and senior policy adviser Stephen Miller will continue to gain stature and influence.

This is dangerous to U.S. interests and international stability.

Hours before Mattis submitted his resignation, Trump announced ill-advised plans to withdraw US troops from Syria. The president also falsely said that ISIS had been defeated. Already Turkey is threatening to invade Syria and kill Kurds, who were aligned with the U.S., Iran and Russia are also emboldened by the surprise U.S. withdrawal announcement, which threatens stability in the Middle East.

Without referencing Syria, Mattis made it clear in his resignation letter that he had strong differences with the president.

“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. 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,” Mattis wrote.

“Because you have the right to a Secretary of Defense whose views are better aligned with yours on these and other subjects, I believe it is right for me to step down from my position,” he said in the letter.

Former Defense Secretary and Maine Sen. William Cohen was among the U.S. leaders who expressed grave concerns about what Mattis’ departure means for the the U.S. and the world. In an interview with the BBC, Cohen noted some of the many times Trump had ignored Mattis’ sage advice: The president discounted the assessment of the entire U.S. national security team that Russian had interfered in the U.S. election system, saying he believed Russian President Vladimir Putin instead. The president ignored advice from Mattis and others against proceeding with negotiations on nuclear arms with North Korea without a specific agreement. Last month, the president sent active troops to the U.S. border to stop a migrant caravan even though their work there was limited by federal law. And, most recently, the troop withdrawal from Syria.

Even though Trump sometimes ignored the advice of Mattis and other seasoned and thoughtful cabinet secretaries and advisers, it was important that they actively worked to educate the president and to change his mind when he was wrong.

With Mattis’ departure, the White House has lost a critical voice of reason and someone who truly understands the value of international alliances, the need for skepticism of bad actors such as Russia and North Korea, and the importance of U.S. leadership — moral, military and diplomatic — in the world.

Mattis, the former Marine Corps general clear-eyed to the dangers and responsibilities the U.S. faces around the globe, no longer sees a place for himself in the Trump administration. That “distressed” even Senate Majority Leader Mitch McConnell, and should give us all pause about the president’s understanding and commitment to America’s leadership role in the world.