North Korea's annulment of non-aggression and denuclearization agreements following unanimously-passed UN Security Council sanctions is, like most actions taken by the North, both troubling and par for the course. In fact, perhaps the only predictable facet of the secretive and volatile regime is its hyperbolic rhetoric, which includes threats of turning Washington into a "sea of fire" and the "final destruction" of South Korea.
Annulment of non-aggression agreements is nothing new on the Korean peninsula. In 2009, the South's pledge to join the Proliferation Security Initiative prompted North Korea to declare that the Armistice between the two countries had been rendered ineffective, and Yonhap News provided a chronology of such statements, showing "a pattern begun in the 1990s... often done in reaction to South Korean and U.S. military moves."
China, as North Korea's greatest ally and largest trading partner, remains the most significant variable in Northeast Asian relations. And while it is encouraging that China voted in favor of the recent sanctions, their enthusiasm in enforcing them remains to be tested. "Although China clearly opposes North Korea’s nuclear tests," notes the Center for Strategic and International Studies, "maintaining the status quo remains its main priority... In the past, China-DPRK trade has increased in the aftermath of UN sanctions."
While at the moment North Korea's threats are rhetorical, keep in mind that 2010 saw both the sinking of the South Korean Navy ship Cheonan and the shelling of the South's Yeonpyoung Island by the North. The threat is real, but even military skirmishes don't necessarily ignite an "all-out war"—precisely what both Koreas (and China) would rather avoid.
Nevertheless, the North's recent nuclear test and current bombast provides an early test for South Korea's newly-elected president Park Geun-hye and her "trustpolitik," that is, the establishment of "mutually binding expectations" that allows for both toughness and flexibility with North Korea. We will find out very soon how this campaign policy manifests itself in praxis.