ARLINGTON, VA - NOVEMBER 09:  U.S. Secretary of Defense Jim Mattis (L) hosts an honor cordon ceremony to welcome China State Councilor and Defense Minister General Wei Fenghe at the Pentagon, on November 9, 2018 in Arlington, Virginia. The two planned to discuss "risk reduction" in an effort to limit the possibility of inadvertent clashes. (Photo by Mark Wilson/Getty Images)
Mark Wilson/Getty Images
ARLINGTON, VA - NOVEMBER 09: U.S. Secretary of Defense Jim Mattis (L) hosts an honor cordon ceremony to welcome China State Councilor and Defense Minister General Wei Fenghe at the Pentagon, on November 9, 2018 in Arlington, Virginia. The two planned to discuss "risk reduction" in an effort to limit the possibility of inadvertent clashes. (Photo by Mark Wilson/Getty Images)
Now playing
04:47
CNN reporter: Mattis is falling on his sword
Rep. Marjorie Taylor Greene (R-GA) speaks during a press conference outside the U.S. Capitol on February 5, 2021 in Washington, DC. The House voted 230 to 199 on Friday evening to remove Rep. Marjorie Taylor Greene (R-GA) from committee assignments over her remarks about QAnon and other conspiracy theories.
Drew Angerer/Getty Images
Rep. Marjorie Taylor Greene (R-GA) speaks during a press conference outside the U.S. Capitol on February 5, 2021 in Washington, DC. The House voted 230 to 199 on Friday evening to remove Rep. Marjorie Taylor Greene (R-GA) from committee assignments over her remarks about QAnon and other conspiracy theories.
Now playing
03:20
Marjorie Taylor Greene lashes out at media after backlash over controversial caucus
AP
Now playing
03:16
Maxine Waters: Jim Jordan is a bully and I shut him down
US Representative Marjorie Taylor Greene, Republican of Georgia, leaves her office on Capitol Hill on February 4, 2021 in Washington, DC.
NICHOLAS KAMM/AFP/Getty Images
US Representative Marjorie Taylor Greene, Republican of Georgia, leaves her office on Capitol Hill on February 4, 2021 in Washington, DC.
Now playing
03:51
Marjorie Taylor Greene launching 'America First' caucus
US President Joe Biden delivers remarks on Russia at the White House in Washington, DC on April 15, 2021. - The United States announced sanctions and the expulsion of 10 Russian diplomats Thursday in retaliation for what Washington says is the Kremlin's US election interference, a massive cyberattack and other hostile activity.
JIM WATSON/AFP/Getty Images
US President Joe Biden delivers remarks on Russia at the White House in Washington, DC on April 15, 2021. - The United States announced sanctions and the expulsion of 10 Russian diplomats Thursday in retaliation for what Washington says is the Kremlin's US election interference, a massive cyberattack and other hostile activity.
Now playing
02:22
White House backtracks on refugees decision after criticism
Biden speaks from the Treaty Room in the White House on April 14, 2021 in Washington, DC.
Andrew Harnik/Pool/Getty Images
Biden speaks from the Treaty Room in the White House on April 14, 2021 in Washington, DC.
Now playing
02:44
'National embarrassment': Biden reacts to mass shootings
MOSCOW, RUSSIA - APRIL 15:  Russian President Vladimir Putin (L) talks to talks to Foreign Minister Sergey Lavrov during a meeting with Uzbekistan President Islam Karimov in the Kremlin on April 15, 2013 in in Moscow, Russia. Karimov is on a state visit to Russia. (Photo by Sasha Mordovets/Getty Images)
Sasha Mordovets/Getty Images
MOSCOW, RUSSIA - APRIL 15: Russian President Vladimir Putin (L) talks to talks to Foreign Minister Sergey Lavrov during a meeting with Uzbekistan President Islam Karimov in the Kremlin on April 15, 2013 in in Moscow, Russia. Karimov is on a state visit to Russia. (Photo by Sasha Mordovets/Getty Images)
Now playing
02:07
Russia to expel 10 US diplomats in 'tit-for-tat response' to Biden sanctions
Now playing
03:10
Avlon: Here's what we know 100 days since the Capitol riot
A Russian flag flies next to the US embassy building in Moscow on October 22, 2018. - US national security advisor John Bolton is in Moscow holding meetings with senior Russian officials following Washington's weekend announcement of withdrawal from the Cold War-era Intermediate-Range Nuclear Forces Treaty, known as the INF. (Photo by Mladen ANTONOV / AFP)        (Photo credit should read MLADEN ANTONOV/AFP via Getty Images)
Mladen Antonov/AFP/Getty Images
A Russian flag flies next to the US embassy building in Moscow on October 22, 2018. - US national security advisor John Bolton is in Moscow holding meetings with senior Russian officials following Washington's weekend announcement of withdrawal from the Cold War-era Intermediate-Range Nuclear Forces Treaty, known as the INF. (Photo by Mladen ANTONOV / AFP) (Photo credit should read MLADEN ANTONOV/AFP via Getty Images)
Now playing
02:17
Political scientist: US-Russia relations are in the toilet
WASHINGTON, DC - JUNE 13: Rep. Michelle Lujan Grisham (D-NM) speaks during a news conference on immigration to condemn the Trump Administration's "zero tolerance" immigration policy, outside the US Capitol on June 13, 2018 in Washington, DC. (Photo by Toya Sarno Jordan/Getty Images)
Toya Sarno Jordan/Getty Images
WASHINGTON, DC - JUNE 13: Rep. Michelle Lujan Grisham (D-NM) speaks during a news conference on immigration to condemn the Trump Administration's "zero tolerance" immigration policy, outside the US Capitol on June 13, 2018 in Washington, DC. (Photo by Toya Sarno Jordan/Getty Images)
Now playing
02:39
Governor settles with former campaign staffer who accused her of sexual mistreatment
pool/cnn
Now playing
01:56
Hear what Dr. Gupta said when Cruz went maskless before
Now playing
02:30
Biden's decision to withdraw from Afghanistan is personal for this lawmaker
President Joe Biden speaks from the Treaty Room in the White House on Wednesday, April 14, 2021, about the withdrawal of the remainder of U.S. troops from Afghanistan.=
Andrew Harnik/AP
President Joe Biden speaks from the Treaty Room in the White House on Wednesday, April 14, 2021, about the withdrawal of the remainder of U.S. troops from Afghanistan.=
Now playing
02:10
Why Biden made his Afghanistan announcement in this particular room
U.S. President Joe Biden speaks from the Treaty Room in the White House about the withdrawal of U.S. troops from Afghanistan on April 14, 2021 in Washington, DC. President Biden announced his plans to pull all remaining U.S. troops out of Afghanistan by September 11, 2021 in a final step towards ending America's longest war.
Andrew Harnik/Pool/Getty Images
U.S. President Joe Biden speaks from the Treaty Room in the White House about the withdrawal of U.S. troops from Afghanistan on April 14, 2021 in Washington, DC. President Biden announced his plans to pull all remaining U.S. troops out of Afghanistan by September 11, 2021 in a final step towards ending America's longest war.
Now playing
01:03
Biden: It's time to end the forever war
Kinzinger
CNN
Kinzinger
Now playing
05:56
What Republican lawmaker fears after US troops leave Afghanistan
Rep. Matt Gaetz, R-Fla., questions witnesses during a House Armed Services Committee hearing on Capitol Hill, Wednesday, April 14, 2021, in Washington.
Manuel Balce Ceneta/AP
Rep. Matt Gaetz, R-Fla., questions witnesses during a House Armed Services Committee hearing on Capitol Hill, Wednesday, April 14, 2021, in Washington.
Now playing
02:59
Women detail late-night parties with Gaetz
(CNN) —  

Lots of Cabinet officials – and I mean LOTS – have either resigned or been fired by President Donald Trump in his first 20 months in office. But no one has quit in quite the same way as Defense Secretary James Mattis did Thursday night.

“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 position,” Mattis wrote to Trump in a pointed two-page letter that he hand-delivered to the commander in chief earlier Thursday afternoon.

And then, this:

“One core belief I have always held is that our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships. While the US 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.”

In that single paragraph, which sums up the sentiment of the longer letter, Mattis rejects out of hand Trump’s “America First” foreign policy that had led to clashes with a slew of longtime allies – from Germany to Britain to Australia and back. From Trump’s decision to pull out of the Paris climate accord to his recent announcement that the United States would be leaving Syria – a decision with which Mattis disagreed – Trump has shown time and time again that he rejects the post-World War II alliance structure that presidents, Republican and Democratic, have relied upon for more than seven decades.

It’s impossible to read Mattis’ letter as evidence of anything but a man who had been pushed well past his breaking point and, finally, snapped. Faced with a President who made enemies of our allies and allies of our enemies – no one has arguably been happier with the Trump presidency than Russian President Vladimir Putin – Mattis decided to walk away.

Which is remarkable, both by itself and when you consider the broader context of how and where Mattis fit into the Trump Cabinet and administration.

In a vacuum, Mattis was widely regarded by those familiar with his approach to the office as someone who simply would not walk away without a very good reason. A much-decorated military general, Mattis was part of a group within the Trump administration – that once included chief of staff John Kelly and Secretary of State Rex Tillerson – who viewed their roles in the administration as a duty to the country.

Step back and you see Mattis’ departure as a sort of bookend to Tillerson’s removal as secretary of state, with Kelly’s removal sandwiched in between. Together, those three men were cast – by Trump and his allies – as the crown jewels of the administration, men who had long experience in Washington and around the world and who, together, would form a shield of sorts against Trump’s worst instincts in diplomacy and foreign policy.

Trump tired of Tillerson first. Once enthralled by Tillerson’s time spent as a titan of industry (he was the CEO of ExxonMobil), Trump came to view Tillerson as someone who didn’t understand that his job was to execute the vision of the chief executive – period.