The last member of an informal alliance of top Trump officials with enough swat or stature to stand up to President Trump — the Committee to Save America, as Jim VandeHei and I called these officials 16 months ago — resigned in epic fashion.

• Unlike most others, who pretended to leave on fine terms, Defense Secretary Jim Mattis bailed with a sharp, specific, stinging rebuke of Trump and his America-first worldview. 
• Mattis wrote: "My views on treating allies with respect and also being clear-eyed about both malign actors and strategic competitors are strongly held."
• And the general drops the mic: "Because you have the right to have 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 positions."
• Back when Trump first took office, he had bragged: "[M]y generals ... are going to keep us so safe ... If I’m doing a movie, I pick you, General Mattis."

It was a historic letter and a historic moment capping a historic day, one you could easily see filling a full chapter of future books on the Trump presidency. 

The wheels felt like they were coming off the White House before Mattis quit. 

• The spiral began Wednesday when Trump saw conservative media turn on him when he appeared to be caving on funding for the border wall in order to avoid a government shutdown.
• Trump then announced he was keeping a different campaign promise: withdrawing U.S. troops from Syria. And yesterday, word leaked that he had ordered a drawdown from Afghanistan.
• "[T]he president was super pissed and [conservatives] have him all whipped up ... [H]e is seething at the media reports of him retreating," a Republican lobbyist emailed.
• An outside adviser added: "What triggered Trump on Syria was giving up on the wall."
• By midday, the wall was back and Trump was telling congressional leaders he was prepared to allow a partial government shutdown tomorrow night.

The backdrop ... Spooked by Trump's actions and statements, Wall Street is on track for its worst year in a decade — since the financial crisis of 2008.

Scoop: As a sign of the mood inside, officials at both ends of Pennsylvania Avenue tell us that Trump is complaining about his incoming chief of staff, Mick Mulvaney, in conversations inside the West Wing and with Capitol Hill.

• Trump asked one trusted adviser: "Did you know [Mulvaney] called me 'a terrible human being'" back during the campaign?
• We're told that Trump was furious when the slight surfaced in a two-year-old video right after he promoted Mulvaney. (A spokeswoman says that was before Mulvaney met Trump.)

An outside adviser to Trump told me as the president's "landmark day of chaos" unspooled: "He is straddling the political precipice."

• Why it matters: Mattis was the last to go — after outgoing White House chief of staff John Kelly, former-Secretary of State Rex Tillerson and economic adviser Gary Cohn — of the officials sometimes called the “axis of adults."
• As captured by cable news ... MSNBC: "TRUMP CHAOS" ... CNN: "DEFENSE SECRETARY QUITS ... AS GOVERNMENT SHUTDOWN LOOMS AND FINANCIAL MARKETS TANK" ... Fox News: "SHUTDOWN SHOWDOWN."

Reality check from Jonathan Swan: Trump was never going to adopt the establishment consensus that a strong U.S. military presence would be required for the foreseeable future in Afghanistan and Syria. Trump has never felt that.

• As described by aides, Trump — like many who cheer him on at rallies — views the Middle East as a collection of barely distinguishable "shithole countries."
• Trump believes they are probably beyond saving, and certainly not worth the U.S. trying.
• He wants out. And anyone who hasn’t figured that out by now is smoking some powerful stuff.

Be smart: Thursday was one of the most remarkable days, tucked inside one of the most remarkable weeks, capping one of the most remarkable months in modern presidential history. 

• Scary thought: It’s hard to find people around Trump or in Republican politics who don’t think things could get worse. 
• P.S. NBC News reported last night the Mueller report will drop as soon as mid-February.

Another reason this is a historic week — and what President Trump should really worry about — is that lots of different Republicans have been turning on him over different topics.

• In an unusually harsh statement, Senate Majority Leader Mitch McConnell said he was "distressed" about Mattis' departure: "It is regrettable that the president must now choose a new Secretary of Defense. But I urge him to select a leader who shares Secretary Mattis's ... principles."
• Sen. Ben Sasse (R-Neb.) said after the Mattis announcement: "This is a sad day for America."

A former Trump aide who asked to be described as "a Trump ally" told me that the sudden wave of criticism from the Hill over Syria and Mattis should scare the president because he would desperately need these lawmakers' support during a possible impeachment battle.

• "Once Republican lawmakers start rebuking the president publicly like this over policy, it makes it easier for them to say: 'It's not just Mueller or ethics. There are other concerns.' Then it's a slippery slope."

Bloomberg columnist Eli Lake: "Donald Trump may not know it yet, but his presidency is collapsing."

• "As long as [Mattis] served the president, reluctant Republicans could point to the Pentagon and say: If Mattis supports Trump, then so do I. They can no longer do that."

Be smart: We talked all day yesterday with Republican officials, operatives and advisers who are truly scared for America.

• But it's telling that few have the courage to say it publicly.

What to watch: There’s a working assumption inside the White House that Trump will be impeached by the House. He would then need a rock-solid base of 34 Republican senators to refuse to remove him from office (which takes 67 votes).

• So nothing matters more to Trump than keeping his base happy and loyal. 

Ivanka Trump applauds as President Trump takes the stage in the South Court Auditorium of the Eisenhower Executive Office Building to sign the farm bill.

House Majority Leader Kevin McCarthy and House Speaker Paul Ryan leave the West Wing after talking briefly to reporters about their meeting with Trump.

In the rain outside the West Wing, Sarah Huckabee Sanders talks to reporters about a possible shutdown.

Kevin McCarthy is met by reporters at the Capitol after he returned from the White House.

"A senior Justice Department ethics official concluded acting attorney general Matthew G. Whitaker should recuse from overseeing special counsel Robert S. Mueller III’s probe, ... but advisers to Whitaker recommended the opposite and he has no plans to step aside," the WashPost's Devlin Barrett and Matt Zapotosky report:

• Late yesterday, "the Justice Department formally notified Congress of Whitaker’s decision not to recuse, writing in a letter that while an ethics official felt he should do so to avoid to appearance of a conflict, that official could not identify a precedent for such a recusal."

Why it matters: "The advice to stay away from the Mueller probe underscores the ... deep distrust — within Congress and in some corners of the Justice Department — surrounding Whitaker’s appointment ... until the Senate votes on the nomination of William P. Barr to take the job."

• Read the letter to Congress saying Whitaker won't recuse himself.

The VIX — Wall Street's top measure of volatility (stock swings) — is set for its biggest annual surge on record, per Bloomberg:

• "Fears that growth in corporate earnings and the global economy are slowing have beset investors globally."
• "One consequence of this resurgence of volatility: it’s brought more stock trading. Volume on U.S. exchanges has climbed to an average 7.2 billion shares daily this year, up 11 percent from 2017."

AP's top 10 stories of the year, as voted by U.S. editors and news directors in a poll first conducted in 1936, when editors chose the abdication of Britain’s King Edward VIII:

1. Parkland school shooting
2. Trump-Russia probe
3. MeToo
4. Mass shootings, including Pittsburgh
5. Midterms
6. Immigration
7. Kavanaugh hearings
8. California wildfires
9. Climate change
10. Khashoggi killing

Digital detox: No-cell hotels ... "Can you take a vacation from your cell phone? A growing number of hotels will help you find out. Some resorts are offering perks, like snorkeling tours and s'mores, to guests who manage to give up their phones for a few hours," AP's Dee-Ann Durbin writes:

• "Some have phone-free hours at their pools; others are banning distracting devices from public places altogether."
• "On Oct. 1, Wyndham Grand’s five U.S. resorts began offering prime spots by the pool, free snacks and the chance to win return visits when guests put their phone in a soft, locked pouch. The phones stay with the guests, but only hotel staff can unlock the pouches."

"Hotels that limit cellphone use risk losing valuable exposure on Instagram or Facebook. But they say the policies reflect their mission of promoting wellness and relaxation."