The anxious armistice between U.S. President Barack Obama and president-elect Donald Trump came to an abrupt halt Wednesday and put the United States on course for the rockiest presidential transition in at least six decades. Cracks in the relationship have been evident for weeks, but the fissure between the two broke into the open when Mr. Obama’s Secretary of State defended the American position in the United Nations on the issue of Israeli settlements and when Mr. Trump went to his Twitter account and banged out an unambiguous end to the surface niceties the two have laboured to present to a divided American public.