The unbreakable alliance between China and North Korea that was "sealed with blood" during the Korean War is a relic of the Cold War during the height of communism. What was once seen as a vital buffer zone between China and western influence on the Korean peninsula has become a buffer of questionable value given China's major shift in its trade relationship with the west and its moderating political ideologies over the past three decades. The U.S. and South Korea each rank in China's top five trading partners with respect to both imports and exports. The trade relationship China has with North Korea is relatively insignificant. On the spectrum of political ideology China has also drifted away from the hardline socialist track maintained by North Korea by virtue of facilitating ever increasing economic relations with the west. China does not support North Korea's nuclear weapons program, which places regional stability at risk, and cooperated with the other members of the UN security counsel in the most recent round of sanctions enacted in response to N. Korea's latest nuclear test. China has recently mobilized military forces along the border with North Korea but the lack of a clear message of military support in opposition to South Korea or the U.S. may indicate preparations to regulate the flow of refugees into China in the event of military exchange on the Korean Peninsula rather than intervention on behalf of Pyongyang.
The North Korean regime is likely in the most fragile state that it has been in decades. It's economy has steadily declined over the years as a result of poor management by central planners combined with increasingly painful sanctions imposed in response to its nuclear weapons program. The costs of North Korea's nuclear program have come not only in the form of increased isolation from the global community but have also likely resulted in neglect of its conventional forces. With an estimated GDP of $40 billion North Korea's nuclear weapons program has likely forced painful budgetary sacrifices over the last decade especially if you consider the cost of the United States' development of a nuclear bomb (estimated at over $400 billion). The death of Kim Jong Il and succession of his 28-year-old son Kim Jong Un also threatens the control of the Kim family regime which began following the Korean war with the reign of Kim Il Sung, the grandfather of Kim Jong Un. While Kim Il Sung is deified in North Korea as the "Eternal President," adulation of his progeny has been less epic. The recent provocations from the current regime whose cryptic propaganda videos have recently been circulating on YouTube aren't going very far in garnering external sympathy for Pyongyang, and make justifying any support China is still willing to offer to North Korea all the more difficult.
There are a number of wild cards that pose significant challenges to accurately predicting the direction this stand off will take. Most notably, recent leadership changes in three of the principle states. Kim Jong Un, the youngest son of Kim Jong Il, was selected to succeed his father as the head of state in North Korea in the spring of 2012. Park Geun Hye, the first female president of South Korea assumed office in February 2013. Xi Jinping assumed the office of General Secretary of the Communist Party of China in November of 2012 and is expected to continue the trend of moderating the party. Although American President, Barack Obama, is in his second term of office defense spending and budgetary concerns in general are major factors temporing the American position with regard to North Korea, however a weak response from Washington would likely hamstring President Obama's goals of disarmament and non-proliferation in other key areas of interest, most notably Iran.