Trump tests the resolve of his Republican backers as he ends the year in chaos
President Donald Trump -- seen here in Buenos Aires during the G20 summit. (AFP/File / SAUL LOEB)

Trump’s presidency has lurched from crisis to crisis since he took office less than two years ago, but Thursday was a landmark day of chaos that appeared to test the resolve of even senior Republican backers in Washington.

Defense Secretary James Mattis, a widely respected figure seen as a stabilizing influence inside the administration, handed in his resignation after arguing with Trump over foreign policy in a White House meeting.

Mattis then released a letter that showed fundamental policy differences between the two men and implicitly criticized Trump’s disregard for allies abroad.

Also on Thursday, Trump resisted pressure to stand down from a decision to withdraw U.S. troops from Syria, made plans to pull American forces out of Afghanistan and pushed the U.S. government toward a shutdown over funding for a border wall.

To top it all off, U.S. share prices tumbled as investors worried the looming shutdown, slower economic growth and the Federal Reserve’s projections for more interest rate hikes next year.

Even some of Trump’s friends showed deepening worry about where his administration is heading at the halfway point of his term.

Senator Lindsey Graham, a close ally and frequent golf partner, on Thursday praised Mattis, publicly urged Trump to reconsider the Syrian pullout and warned that withdrawing troops from Afghanistan could ultimately lead to another attack on America similar to the one on Sept. 11, 2001.

“I believe you are on course to make the same mistake President (Barack) Obama made in Iraq ... It will turn out no better for you than it did for him.” Graham tweeted of the Syria move.