The world is a complex place, its problems not usually susceptible to simplistic solutions, let alone knee-jerk reactions. We may not be fans of decisions that have enmeshed the United States military in wars big and small in the Middle East and elsewhere around the world, but now that we are there, we value thoughtful and sober assessments of what we do there and how we get out.

Thoughtful and sober are words people who know Defense Secretary Jim Mattis use to describe him, but not his tempestuous commander-in-chief. And that’s why we and so many others are dismayed and worried by Mattis’s resignation Thursday after clashing with President Donald Trump over the abrupt withdrawal of U.S. troops from Syria and after two years of deep disagreements over America’s role in the world.

Mattis, perhaps the most respected foreign policy official in Trump’s administration, will leave by the end of February after two tumultuous years struggling – by most accounts – to soften and moderate the president’s hardline and sometimes sharply changing policies involving U.S. military power – often made without consulting or warning allies.

The phrase often heard as news of the Mattis resignation spread was “adult in the room” – and they weren’t talking about the president.

Mattis’s departure was immediately lamented by foreign policy hands and lawmakers on both sides of the aisle, who viewed the retired Marine general as a sober voice of experience in the ear of a president who had never held political office or served in the military. Even Trump allies expressed fear over Mattis’ decision to quit, believing him to be an important moderating force on the president.

Other Republican leaders like Florida Sen. Marco Rubio are lamenting Mattis’ loss because they don’t trust Trump, fearing our military and foreign policy will be guided by hotheads like advisor Stephen Miller and National Security Adviser John Bolton or conservative talking heads from Fox News.

“It makes it abundantly clear that we are headed toward a series of grave policy errors which will endanger our nation, damage our alliances and empower our adversaries,” Rubio said.

In his resignation letter, Mattis noted his “core belief” that American strength is “inextricably linked” with the nation’s alliances with other countries, a position seemingly at odds with the “America First” policy of the president who has attacked –early and often – our traditional allies and alliances like NATO and the European Union. The defense secretary also said China and Russia want to spread their “authoritarian model” and promote their interests at the expense of America and its allies.

“While the U.S. remains the indispensable nation in the free world, we cannot protect our interests or serve that role effectively without maintaining strong alliances and showing respect to those allies,” Mattis wrote.

But Trump seems to want nothing more than to cozy up to Russia and doesn’t stand with our allies against Chinese and Russian influence.

The announcement of the general’s resignation came a day after Trump surprised U.S. allies and members of Congress by announcing the withdrawal of all U.S. troops from Syria. He declared the terrorists group Islamic State dead. He seems the only person who thinks this. Trump’s decision has been sharply criticized for abandoning America’s Kurdish allies, who may well face a Turkish assault once U.S. troops leave, and had been staunchly opposed by the Pentagon, which feels our troops are still needed to help keep in a box, Islamic State fighters who once overran nearly all of Syria and Iraq.

Mattis, an intelligent, battle-hardened retired Marine, has brought military calm and judgment to the administration’s often chaotic national security decisions, and we worry that his replacement will be more of a “yes man” because we’ve learned the commander in the White House prefers to shoot from the hip, based on his own self-interests of the moment, with minimal knowledge or interference from the experts.

So, with the departure from the White House of the last of “the generals” that Trump once touted, another check on his impulsiveness will be gone. The president’s allies will see that as a good thing: Now perhaps the Pentagon can speed up the ban on transgender people serving in the military, roll out that ego-boosting military parade, and ship tanks to repel refugees at the Mexican border, which Trump aide Stephen Miller insists is the real military threat to America, not ISIS.

As we said, American use of hard military power around the world may not be the best answer to our or the world’s problems, but every day we need thoughtful, careful, informed decision-making, which Mattis represented.

We see his departure fraught with danger.