Trump is forcing out Defense Secretary James Mattis by New Year's

AP Photo/Pablo Martinez Monsivais, File

In this Nov. 9, 2018, file photo, Defense Secretary Jim Mattis waits outside the Pentagon.

President Trump announced that Secretary of Defense James Mattis will leave his position by January 1. Patrick Shanahan, Mattis's deputy, will take over as the acting defense chief.

Mattis announced his resignation from the Defense Department on Thursday over President Trump's decision to withdraw United States troops from Syria and halve the number of troops in Afghanistan. Mattis originally said he would stay in his position until February to ensure an orderly transition.

In his resignation letter, Mattis blasted Trump for mistreating allies.

"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," he wrote.

Before beginning his position as deputy secretary of defense in June 2017, Shanahan was an executive at the aircraft and weapons manufacturer Boeing. He does not have any prior government experience.

This story is developing.