U.S. Secretary of Defense Secretary James Mattis has resigned his post in disgust due to irreconcilable differences in policy with president Donald Trump. His resignation takes effect on Feb. 28, 2019.

In his resignation letter delivered to Trump, the former U.S. Marine General put his reasons for quitting more diplomatically. He said Trump has "a right to have a Secretary of Defense whose views are better aligned with yours."

Mattis, 68, served as Trump's Secretary of Defense since the start of the Trump administration in January 2017. In his astonishing resignation letter, Mattis indirectly excoriated Trump for antagonizing America's traditional allies while cozying up to its deadliest of enemies, especially Russia.

Mattis said he's remained true to a long-held "core belief" that "our strength as a nation is inextricably linked to the strength of our unique and comprehensive system of alliances and partnerships."

Without maintaining those alliances, we cannot protect our interests or serve the role of an "indispensable nation in the free world."

Mattis said his 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 warned the US should resolutely resist attempts from China and Russia to "shape a world consistent with their authoritarian model," and that the U.S. should protect the "international order."

The well-read Mattis, a voracious reader, makes it clear Trump does not share his beliefs.

He said because Trump has 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. The end date for my tenure is February 28, 2019."

Mattis' resignation letter comes only a day after Trump's controversial plan to withdraw U.S. troops from Syria. Trump's sole reason: he made the promise in the presidential campaign.

Trump has also ordered the Pentagon to look into plans for withdrawing troops from Afghanistan.

The move to unilaterally quit Syria, without consulting any of America's allies on the ground, especially the Kurds, met with angry criticism from a number of Trump's allies in Congress.

Sen. Marco Rubio (R-FL) tweeted that Mattis' loss "makes 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."