US Now Obligated to Fight If North Korea Attacks South

It's official: If North Korea provokes the South, the US is now obligated to help Seoul respond. The military today announced Friday's signing of a mutual defense agreement that the two countries have been working on since the North fired on a South Korean island in 2010, the New York Times reports. The deal requires the US military to, in the Times' words, "fight to defend" South Korea should the Korean Peninsula become a war zone, and outlines a variety of potential North Korean moves along with the allies' planned responses.

Per the "nuclear umbrella," the US has already promised to aid the South against a nuclear threat, but the latest move requires Washington to also respond to low-level incidents. That's significant, because until now the US did not need to get involved in "minor skirmishes," the BBC reports. Both parties say it will deter Pyongyang from such local provocations. But neither side is saying much in terms of specifics, including an answer to the major question: When would US troops be forced to step in? (More Pyongyang stories.)