U.S. Secretary of Defense James Mattis warned North Korea Friday that a nuclear attack on any of America’s allies would trigger an “effective and overwhelming” response. Mattis, speaking in Seoul, apparently sought to reassure the region’s leaders in the wake of President Trump’s many isolationist statements and veiled threats against U.S. allies. Trump at various points during his campaign threatened to withdraw U.S. forces from South Korea and from Japan unless those countries ramped up their financial support. The Mattis tour, the first abroad by a senior Trump administration official, came as reports emerged in South Korea that North Korean dictator Kim Jong Un had fired his top minister for state security. Kim Won Hong, who led the Hermit Kingdom’s notorious secret police force, was removed in mid-January on charges of corruption and human-rights abuses, said Jeong Joon-hee, a spokesman for the South’s unification ministry. The report of the ouster not been independently verified, but moves by the regime are rarely officially announced.