This is purely speculation, but just an Idea of the Worst Case Scenario:
July 27, 2017: On the anniversary of the end of hostilities in the Korean War, North Korea successfully tests a 50 kT nuclear device small enough to fit on a Medium Range nuclear weapon capable of reaching at least as far as Japan and potentially as far as Alaska or Hawaii. The test comes after a series of warnings by the US and China that a deliverable weapon would be considered a direct threat to global security.
August 5, 2017: The US and China coordinate joint nuclear strikes against Musudan-ri and Tongchang-ri, and conventional bombing campaigns against North Korea's nuclear research facilities. Half a million American and South Korean ground troops move through the DMZ after a series of heavy bombing runs clears several paths of North Korean forces and land mines. China sends another half a million men through the North of the country.
August 7, 2017: The ground war in Korea continues, led largely