Since the 1950's the US has been in a cease fire with North Korea, we never signed a peace treaty. Think of this like the first Gulf war when we singed a cease fire with conditions. This is the reason why we keep thousands of troops along the Korean border in case the North ever decided to revoke the cease fire. The thing is the North has done this on numerous occasions, but the US hasn't done more than plea with the Chinese to take care of this problem. This has given the North the message that the US and the West have no real desire or heart for war which could be what actually escalates this even farther. Every time the North does something a little more grave than the last. Remember a few years ago North Korea launched a missile into the ocean, now they're firing artillery rounds at a South Korean island.
The biggest reason we had such a hard time during the Korean war was because the Chinese and Russians were supplying the North with tanks and soldiers. This made it very hard for the US to unite the North and South. This is also what plays into the current problems. Since the US knows that China is backing the North and can control them more than we can because China is their sole support. China hasn't really showed a desire to reign in North Korea, but through concessions have been persuaded to act on our behalf.
With the seemingly shift in the balance of world power shifting from the US to China, China could very well be telling the North that nothing will happen to them. I think the North also sees this and may step up its efforts to unify the North and South. With the large US debt and China holding the largest portion of foreign debt for the US this has left us open to being forced to step back.
I think there is a real miscalculation here though by the North and the Chinese. If the North provokes the South into war, the Japanese would be forced into this as well because it would be in their best interest. If China controls 2 of the 3 top Asian economies in the world this could mean serious trouble especially since China and Japan have a serious hatred of each other as bad as the Arabs and the Jews. Also, South Korea is 1 of only 2 free economies in the region and may have other ties. Don't forget the US has treaties with the South and Japan to defend them. Recently, because of the strain on the US military, Japan has started militarizing itself in order to protect itself as well.
There are three key factors that need to be taken into consideration about this incident that weren't normally part of the equation. First would be the shift in global power for the Chinese over the US, Secondly North Korea knows this and may be emboldened, and lastly with a global economic collapse a world war could bring recovery like WWII did or the fact of the global economy in near shambles most countries will be unable to react until the war is on their doorsteps.
I think we as a nation need to really pay attention to this because if things aren't handled right or we don't make a strong enough statement this could lead to the beginning of WW III, but this is just my point of view based upon 30 years of experience in life and my following of the political arena the last 10 years along with my love for history. Depending on how we handle this situation we could get out of this without anything escalating, but considering President Obama's track record with other issues I'm not sure that I have faith this will work out favorably for the US.