Reporting by CBS News' David Martin

The Pentagon has been ordered to start planning a major drawdown of roughly 7,000 troops in Afghanistan, a senior defense official tells CBS News correspondent David Martin. That's half of the 14,000 U.S. troops still deployed there now.

The news broke on the same day President Trump announced Secretary of Defense James Mattis will be leaving the administration. In an unusual letter, Mattis wrote that the president needs a defense secretary more aligned with his perspective and expressed concerns over America's role on the international stage and its treatment of U.S. allies. The news also comes as Mr. Trump has acknowledged he intends to pull troops from Syria, a move that sparked intense backlash even from strong allies of the president.

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The timeline for drawing down troops in Afghanistan is still unclear. In Syria, Mr. Trump ordered an immediate withdrawal.

On Thursday night, top White House officials backed away from connecting Mattis' departure with any movements in Syria or Afghanistan. One White House official did say Thursday night that Mattis and Mr. Trump had differences over what to do in Syria.

"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 said in the letter he wrote to Mr. Trump.

— Reporting contributed by Kathryn Watson