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 by the South Koreans, while the US and China provide air and naval support, with a degree of ground operations against nuclear enrichment facilities not taken out in the bombing campaign. North Korea shells Seoul and the Sino-American coalition bombs Pyongyang. Congressional investigations into the Trump administration are halted.
August 8, 2017: The shelling of Seoul seriously harms South Korea's economy while the fallout from the nuclear strikes leads to an economic panic in Japan. In the US and China, fear over nuclear war leads to a market panic, exacerbating existing market worries in the financial sector and retail economy, sending the US into a recession that disproportionately impacts the already fragile economies of Europe and China.
September 24, 2017: With global demand for exports lower than at any time since the Crash of 08', and the European financial sector collapsing from defaults on non-performing loans, Germany's economy stands in ruins. Angela Merkel's Christian Democratic Union faces a stunning defeat at the hands of the SPD and the Left, with Martin Schulz becoming the new Chancellor.
March 3, 2018: The remaining leadership of the DPRK Army surrenders to the Chinese. While the war officially ends, with a death toll in the millions (mostly DPRK forces and residents of Seoul), some North Korean army units continue to attack villages and South Korean military positions, however their supplies are extremely limited.
November 6, 2018: While many progressives remain outraged over the apparent indefinite closing of the investigation into the Trump administration's ties to Russia, Conservatives are united and energized by what is seen as an example of President Trump's leadership skills, despite the recession. The Republicans gain 3 new seats in the Senate and hold the House.
January 20, 2019: President Trump holds what a 1 1/2 Term Inauguration in celebration of the Republican victory, promising to pass a rigorous infrastructure bill to reinvigorate job growth. Efforts to repeal Obamacare have largely been put on hold. The US response to the Korean humanitarian crisis that follows the war is placed largely on South Korea while the US and China agree to provide aid to areas impacted by radioactive fallout, which has spread as far south-east as Osaka (although it is believed to be within acceptable levels). During the war famine swept through North Korea killing at least a million people, mostly civilians, and particularly those in the Hoeryong concentration camp, of which only a few dozen survive the war after resorting to cannibalism.
February 2, 2019: A post-war summit is held in Manila between the US, China, and South Korea, with Japan and Russia attending as observers. An agreement is made to unify the peninsula, while the US agrees to phase out its military presence in South Korea, keeping positions at Daegu and Kunsan for a period of 10 years, while both parties agree to remove all nuclear positions from the peninsula and the immediate area surrounding it.
April 14, 2019: The insurgency in North Korea stages a suicide attack against the USS Carl Vinson at night. The carrier partially sinks and the crew are forced to abandon ship. President Trump orders air strikes against civilian centers in Korea believed to be harboring the insurgents.
June 1, 2019: The death toll in Korea continues to mount as China pulls its troops back to the border, preferring to stay out of fighting an insurgency.
August 3, 2021: North Korean insurgent activity has largely stopped as their supplies are exhausted. The South Korean economy has still not recovered and the World Bank does not have cash reserves to spare to pay for the reconstruction of the peninsula. The US agrees to invest in its reconstruction, however the assumption is that President Trump's businesses will attempt to buy up the most promising real-estate.