SEOUL, South Korea — North Korea fired hundreds of artillery shells in waters near South Korean border islands on Jan. 5. Last week, it said it no longer regarded the South as inhabited by “fellow countrymen” but as a “hostile state” it would subjugate through a nuclear war. On Friday, it said it had tested an underwater nuclear drone to help repel U.S. Navy fleets. That new drumbeat of threats, while the United States and its allies have been preoccupied with the wars in Ukraine and the Middle