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 we called these officials 16 months ago — resigned in epic fashion.

The bottom line: 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."

"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."

"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.

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.

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.

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."

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.

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?

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."

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: 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 — 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.

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.