James Mattis resigned as the U.S. defense secretary Dec. 20, with a plan to stay on until Feb. 28, 2019, but President Donald Trump cut short Mattis' tenure by naming an acting defense secretary, effective Jan. 1, 2019.

Deputy Defense Secretary Patrick Shanahan takes over from Mattis on an interim basis.

Mattis cited disagreements with Trump over certain policy matters in his resignation letter, which follows Trump's decision to start withdrawing U.S. forces from Syria and Afghanistan. The move has been criticized not only by Democrats and international allies but also Republicans.

Following a call with Turkish President Recep Tayyip Erdoğan, the U.S. president said Dec. 23 that Turkey will eradicate "whatever is left of ISIS in Syria."

Mattis highlighted the need for maintaining strong relationships with America's allies while calling out China and Russia as those countries whose strategic interests clashed with the U.S.'s, stressing that the country's approach in dealing with such countries must be "resolute and unambiguous."