This is not business as usual: South Korea and the U.S. have called off joint military exercises due to soaring tensions between Seoul and Pyongyang. As we reported yesterday, North Korea and South Korea fired artillery shells at one another this week across the DMZ. The North started the shelling, saying it was provoked by loudspeakers placed at the border to blast anti-Pyongyang propaganda into its territory, a tactic Seoul once employed regularly but hadn’t used in 11 years.South Korea, which argues that it was merely responding to the deaths of two of its soldiers earlier in August by allegedly North Korean landmines, then counter-volleyed with dozens of shells. Nobody on either side is reported to have been killed or injured, but in response to the artillery exchange, Kim Jong-un pronounced the North Korean military on the front to be in “a quasi-state of war.”As far as any two Asian countries shelling one another goes, this incident is comparatively unsurprising. Tempers, especially on the Northern side, flare up pretty regularly. Incidents of the DPRK firing shells across the border or at South Korean islands, not to mention its provocative missile and nuke tests and its wildly hyperbolic threats, are par for the course.But this sort of thing does represent a real danger, and Washington and Seoul, neither of whom are particularly apt to bend to North Korea’s whims, are clearly taking the situation quite seriously; the decision to call off the military exercises, which could be antagonizing Pyongyang, makes it look like people in the know think this is more than your average DPRK snit.