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Every day hundreds of South Korean workers make their way across a special border crossing to work at a North Korean industrial complex that is jointly run by both nations. But not today. Authorities in the North have shut down the crossing and blocked Southerners from entering, taking their ongoing feud from the level of idle threats to very real action.

The Kaesong Industrial Complex sits on the North side of the border and is pretty much the only legitimate avenue of partnership between the two nations. Yet, because they've recently made it their mission to antagonize the South at every turn, this move is one of few they have (short of actual war) that can actually hurt their neighbors economically. (But not as much as it hurts the North.)

Authorities in Pyongyang have shut down the complex before, temporarily preventing some South Korean workers from returning home back in 2009. That standoff lasted less than a week, before operations resumed as normal. The park, which opened in 2004, was financed and built by South Korean businesses (the South even provides the electricity) and in return, gives those firms access to more than 40,000 North Korean workers. It's just a few miles north of the Demilitarized Zone and is connected to the South by special rail and road access. 

There were fears that a similar "kidnapping" situation might develop this time, but workers are reportedly being allowed to go home. They're replacements just can't get in. Some companies are keeping their workers there voluntarily to prevent the businesses from shutting down completely. 

After nearly a month of back-and-forth statements and various shows of force, the last couple of days have seen a serious increase in tensions on the Korean Peninsula, as the North has moved from mere words to behavior of real concern. On Tuesday, they announced that they would be restarting their mothballed nuclear reactor with the express purpose of making more plutonium fuel for nuclear weapons. That prompted Secretary of State John Kerry to hold a joint press conference with his South Korean counterpart in Washington to state that "the United States will do what is necessary to defend ourselves and defend our allies, Korea and Japan."

The real danger now is that if the North remains determined to continually assert its strength, it will soon run out of moves that won't do real damage. There's only so many times you can threaten to take on your enemies before you actually have to take them on—or back down. Kim Jong-un has been on the job for only a little over a year and is a newcomer at this game, so there are some in America who are worried he will make a dangerous misstep  Already under crippling economic sanctions, shutting down Kaesong actually hurts the North far more than it hurts the South. When you're dealing with someone routinely acting against their own interests—on a peninsula that is suddenly obsessed with nuclear power—every next move could be the one you're dreading the most.

This article is from the archive of our partner The Wire.