North Korea, the world's most erratic nuclear power, has become the most dangerous as well. Just days after its young new leader, Kim Jong Un, recklessly threatened nuclear attacks against South Korea and the United States, the rogue nation said Monday it has unilaterally scrapped the armistice that ended Korean War fighting in 1953.

North Korea has made similar bellicose threats in the past, and a nuclear exchange would be suicidal. But because North Korea is squeezed economically by tough international sanctions, there is a risk the impoverished nation could turn to selling nuclear weapons or nuclear technology to bad actors and governments around the world. The escalating tensions will command the attention of the Obama administration, which has been more focused on Iran's nuclear ambitions, unrest in the Middle East, terrorism and the war in Afghanistan.

But it is China, North Korea's neighbor and most important trading partner, that must step up to cool those tensions. To avoid chaos on its border and a potential flow of refugees, China must lean hard on its erratic ally not to do anything rash that would spark renewed military conflict.

The situation has gone downhill fast since Feb. 12, when North Korea detonated a nuclear weapon underground -- its third such test since 2006. The United Nations responded by imposing new economic sanctions on the already heavily sanctioned country, where starvation is chronic.

Kim, who took power in December 2011, succeeding his father Kim Jong Il, who ruled for 17 years, threatened nuclear attacks on South Korea and the United States just as those countries began two months of routine, joint military drills on March 1.

The latest escalation in the fast-developing drama came Monday when its state controlled media said North Korea had unilaterally nullified the armistice that ended the Korean War. That agreement ushered in a cold cease-fire that, with the help of U.S. troops stationed on the border between North and South Korea, has defined the status quo at the 38th parallel for 60 years.

There are no signs of new border hostilities. But South Korea also has a new leader, President Park Geun-hye, who is being tested by this war of words just two weeks after her inauguration. She has put her nation's troops on high alert amid fear there could be some new armed skirmish in the offing similar to a 2010 North Korean artillery barrage on a South Korean island that left four dead.

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The United States has provided a nuclear security blanket for South Korea since the armistice. But the United States is cutting its military budget and confronting volatile conditions elsewhere in the world, stoking fear in South Korea that it may no longer be able to rely on the Americans to ensure its security. Public sentiment is reportedly building in South Korea for a nuclear deterrent of its own. It has no nuclear capability and is not likely to develop one anytime soon. But a confrontation between two old military enemies, each with neophyte leaders with something to prove, could be nightmarish.

North Korea has a history of saber rattling to strengthen its bargaining position when sanctions bite. While it may be tempting to laugh off rolling back a Truman-era treaty, neither the United States nor China can ignore an impoverished, insular, nuclear-armed nation.