KOREA/

For a generation, the arc of international events has been mainly positive — the Cold War concluded, the Germanys reunited, apartheid is over. But a few conflicts refuse to end, and one became worse today as North Korea and South Korea exchanged artillery fire, killing two South Korean soldiers. It’s not yet clear how the incident began. Presumably the United States, which has substantial forces in South Korea, Japan and Guam, is at the moment watching closely.