Mattis, who entered office hours after Trump on Jan. 20, is due to arrive Thursday in Seoul, where he will meet with his counterpart, Defense Minister Han Min Koo, amid a swirl of political turmoil. President Park Geun-hye was impeached in December and the constitutional court is reviewing whether to formally end her rule. Later in the week, Mattis is to hold talks in Tokyo with Defense Minister Tomomi Inada and other senior Japanese government officials.