President Trump tweeted Sunday that he had appointed Patrick Shanahan, Mattis's deputy and a former Boeing exec, to serve as the acting Secretary of Defense, effectively confirming reports that he was pushing to terminate Mattis immediately, ahead of the Feb. 28 departure date that Mattis had set out in his letter.

? @realDonaldTrump

I just had a long and productive call with President @RT_Erdogan of Turkey. We discussed ISIS, our mutual involvement in Syria, & the slow & highly coordinated pullout of U.S. troops from the area. After many years they are coming home. We also discussed heavily expanded Trade.