WASHINGTON (AP) — Far from home if not away from election-year politics, US President Barack Obama is returning to the threat to American security that he calls the gravest of all: terrorists getting material for a nuclear bomb. In South Korea, where Obama is headed, the president will join a massive gathering of world leaders whose united goal is to secure nuclear material and prevent it from being smuggled to states or groups intent on mass destruction. Right across the border but not participating: nuclear North Korea, labeled by the White House as "the odd man out." It is brinksmanship with North Korea and Iran, another nation not invited to the summit, that has dominated much of the nuclear debate and that will cast an unquestionable shadow over talks in Seoul. Obama's mission over three days in the South Korean capital will be to show progress — in pressuring North Korea to change its rogue ways and in approaching a lofty goal of locking down nuclear material around the globe by 2014.