His recognition that he could no longer work for an erratic commander in chief who decided to pull US troops out of Syria, apparently without consulting anyone, could lead to a new period of global uncertainty as Trump slips his remaining restraints.

Grave faces on Capitol Hill and the shaken voices of retired military men on cable news reflected the Pentagon chief's renown as more than a decorated warrior, retired four-star general and the most admired Cabinet member.

He is a talisman.

For two years, politicians, foreign policy experts and allied diplomats would quietly confide their belief that as long as Mattis was in the Situation Room, alongside the impulsive Trump, everything would be OK.

Even after the departure of other so-called adults in the room, such as former Secretary of State Rex Tillerson and ex-national security adviser H.R. McMaster, Mattis stayed.

Fears of a clash between the US and Chinese navies in the South China Sea, an opportunistic Russia, meltdowns in the Middle East or a sudden global crisis with Trump at the helm were eased by thoughts of the scholar-general in the chain of command.

Now, he's going. And the world had better prepare for an unchained US President.

Shock and concern

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"There is a lot of concern like I have never seen in my life," Adam Kinzinger, a Republican House member from Illinois, said on CNN, reflecting the sentiments of his colleagues at the events of the last few days.

A senior conservative House GOP member, who supports Trump, told CNN's Jim Acosta: "The wheels may be coming off."

Republican Sen. Marco Rubio warned in a tweet that the Mattis resignation letter made it "abundantly clear that we are headed towards a series of grave policy errors which will endanger our nation, damage our alliances & empower our adversaries."

Such unusual anxiety about Trump among Republicans on Capitol Hill suggested that Mattis might have achieved at least part of his aim in resigning.

Though he addressed his letter to Trump, it was a warning clearly targeted at the lawmakers, especially Republicans, who so admire him. It was also aimed at Americans outside Washington, flagging that the nation is heading down a dangerous path.

Trump's order of an immediate withdrawal from Syria -- the final straw for Mattis -- defied the counsel of his national security experts. Now, it also looks like the start of a period in which America's turmoil could destabilize the globe.

Earlier Thursday, a senior administration official told CNN's Jake Tapper that Mattis was "vehemently opposed" to the Syria decision and a possible Afghanistan troop withdrawal.

The move appeared to have been conducted without consulting allies whose soldiers have fought and died alongside Americans in the 17 years since 9/11.

Trump has every right as commander in chief, given the broad sweep of his constitutional powers and the victory he secured in 2016, to reshape America's posture around the globe. He made promises to bring US soldiers home from foreign battlefields -- a goal shared by many Americans weary of years of war.

Trump's voters wanted someone to shake up Washington and the world and so are unlikely to care that he's spooking the Washington establishment with his populist nationalist approach to the world.

But critics charge that Trump's troop withdrawals will simply hand over tracts of the Middle East and southwest Asia to American's foes such as Iran, Russia, ISIS and the Taliban.

Another Republican who usually supports Trump, South Carolina Sen. Lindsey Graham, warned an Afghan withdrawal would pave the way "toward another 9/11."

A staggering repudiation

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Mattis went further, suggesting that Trump is taking aim at the very foundations of US global power.

In his resignation letter, Mattis suggested Trump's worldview was antithetical to everything he held dear in 40 years in uniform. Those values were not just his own, they are the bedrock of a nation that was a stabilizing force that made the world more safe.

He implied that Trump, who believed Russian President Vladimir Putin over his own intelligence agents about Russian election meddling, and who has cozied up to dictators, such as Kim Jong Un, had got America's friends and enemies confused.

"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," he continued.

In the coming days, the resignation of Mattis is likely to revive sobering questions over whether Trump is fit to be commander in chief and raised the possibility -- reinforced by a report that Trump is now planning an Afghan withdrawal -- that the President plans to engineer a single-handed retreat from world.

"The President has taken a wrecking ball to every pillar of stability and security we have erected over the past 60, 70 years," an emotional William Cohen, himself a former defense secretary, told CNN's Jim Sciutto.

Cohen spoke to Mattis on Wednesday night and said he was distraught over Trump's Syria order, which he viewed as a desertion of America's allies, especially the Kurds.

Paraphrasing his friend's mindset, Cohen added: "'I have never resigned from anything, I have carried the fight in every battle I have been in, but I cannot do this in the name of our country.'"

It is clear that Syria was the last straw for Mattis. But not the only one.

The defense secretary stood by as Trump has appeased Russia, which he sees as a dangerous US foe. He saw the President decide off-the-cuff to halt US military exercises on the Korean Peninsula after being charmed by Kim. Mattis was even forced to explain why Trump deployed troops to the southern border in a political stunt to bolster his hardline message on immigration. And he's seen Trump insult allied leaders to their faces at bitter global summits.

Given such developments, it's no longer absurd to ask questions like whether the President will suddenly decide to pull American troops home from South Korea after decades of keeping the peace or even pulling out of NATO.

"We are in a new world here ... " former Supreme Allied Commander Wesley Clark said on "Cuomo Prime Time." "We've got to come to terms with the character of the President and how he makes decisions."