President Barack Obama is returning to the threat to American security that he calls the gravest of all: the specter of terrorists getting material for a nuclear bomb.

In South Korea, which Obama visits over the next few days, the president will join a massive gathering of world leaders. Their 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 the world's nuclear debate and that will cast an unquestionable shadow over talks in Seoul.

Obama left for Seoul early Saturday.


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