WASHINGTON — Retired Marine Gen. John Kelly has been toughened by war and tragedy.

The veteran of three tours in Iraq has a new mission: to steady the roiling Trump administration — and quiet the friendly fire — as White House chief of staff.

President Donald Trump tweeted that Kelly is “a true star” of the administration. He has been serving as the secretary of homeland security.

The battle-hardened commander is expected to bring a background of military discipline and order to his new duties.

Kelly retired in 2016 as head of U.S. Southern Command before joining the Trump Cabinet. His son, Marine 1st Lt. Robert Kelly, was killed in November 2010 in Afghanistan.