Mattis resignation letter lays out challenges for successor

  • Part of Defense Secretary Jim Mattis’ resignation letter to President Donald Trump is photographed in Washington, Thursday, Dec. 20, 2018. Mattis is stepping down from his post, Trump announced, after the retired Marine general clashed with the president over a troop drawdown in Syria and Trump’s go-it-alone stance in world affairs. (AP Photo/Jon Elswick)

WASHINGTON — The extraordinary resignation letter that Defense Secretary Jim Mattis handed to a surprised President Donald Trump was not just a product of two years of accumulating frustration with an impulsive boss, but an outline of the strategic hazards facing the next Pentagon chief.