(Editor's Note: President Trump announced in a Dec. 23 tweet that Deputy Defense Secretary Pat Shanahan would take over as acting defense secretary on Jan. 1 , assuming the role two months earlier than Secretary Jim Mattis' planned departure on Feb. 28.) Defense Secretary James Mattis' surprise resignation letter references progress on a national defense strategy that has propelled Pentagon efforts on securing cyber operations viewed as crucial to enhanced lethality, along with the letter's pointed reference to differences...
