The defense secretary's decision to quit Thursday was a warning that will ring through history about an impulsive President who spurns advice, disdains America's friends and proudly repudiates the codes of US leadership that have endured since World War II. Mattis stopped Washington in its tracks -- even after months of stunning plot twists in Donald Trump's presidency, and as stock markets plunge, a legal net tightens around the White House and the government is about to shut down.
