The missile launch won't be an attack--it'll simply be a test, albiet one that contravenes the 2006 UN resolution banning the North from testing missiles or nukes. It's another example of the North increasing tensions and attempting to squeeze concessions out of the US.
The North isn't interested in launching a full scale war. The regime in Pyongyang know this would be suicidal. Hundreds of thousands, perhaps millions, of innocent people would die, but ultimately the US and South Korea would prevail. Their training and equipment gives them a huge advantage over the poorly equipped, poorly trained, under-fed and poorly maintained army the North commands. It wouldn't be an easy war, but the US and South together would win.
The North's actions right now aren't really designed for an international audience primarily--sure, they want the attention of Washington, but most of this rhetoric and most of these tests and provocations are aimed at proving Kim Jong-un's military prowess and authority to the Party, the military and wider population in North Korea. Remember that Kim Il-sung had no such reason to provoke the outside world, because the Party spun the stalemate of the Korean War as a victory for the North, legitimising and cementing the the Kim family's rule. Kim Jong-il was also publicly groomed and prepared for leadership and praised as a great military leader for decades, creating an authority that the current young Kim--who was only in the public eye for a matter of months before his father died--sorely lacks. He needs to stamp his authority down and maintain the illusion of ever present threat that keeps the regime in the North ruling. There are unsubstantiated rumours that an attempt on Kim Jong-un's life was made in November of last year, by one of the North's top generals--a general who has now vanished from public life. The North normally espouses violent rhetoric during every biannual wargame by the US and South Korea, but the reason the stakes are higher and the threats more violent, is most likely due to Kim's desperate need to legitimise and cement his rule.
This creates the danger of accidental full scale war. After the loss of 50 South Korean lives in 2010, the current government in Seoul cannot allow armed provocation to go unanswered. Indeed, the South's military will respond to any strikes from the North with overwhelming and disproportionate force, the action recommend in light of the North's last military strikes. The North has thus far gained very little from the escalation of rhetoric. The South and the US will not return to the negotiating table, nor will they engage with the regime directly or increase food aid, which would free Pyongyang to spend more of what little hard currency it has on the continuing development of weapons. If the North cannot win some kind of concession--indeed the provocations over the last few months have even toughened the Chinese stance on North Korea, the opposite effect to that which is desired--then Kim may feel pressured to authorise limited strikes, which invites powerful retribution from the South. Any retribution by the South would demand further action by the North: if Kim were to resist such pressure, he would most likely be removed or even killed. Kim may however claim victory when the wargames in South Korea come to an end, as most of his domestic audience would believe such a tale.
The problem then is that senior military and Party figures will know that little came of their threats, nuclear and missile tests, and that they may conclude the longer they support a leader unwilling to use their deteriorating arsenal, the weaker their negotiating position becomes. In the long term, the North's army becomes less formidable every day, and in the eyes of the Party and top military figures, this opens the path to their eventual ruin and the triumph of the South and the US. This is one of the key motivations behind the drive to establish a nuclear arsenal--such weapons on the peninsula could attack key US allies and some of the most densely populated urban areas in the world, wreaking untold havoc, which in turns reducing the importance of the North's deteriorating conventional arsenal. If the Party and military conclude that with Kim in power, the North will be unable to win further concessions from the international community that free more resources to devote to the nuclear programme--in their eyes, the ultimate insurance in the continuing rule of the Party and freedom from the threat of outside intervention--then Kim may be removed, or encouraged to launch an attack on Seoul. If the North were to capture Seoul, the Party's thinking is that its negotiating position becomes much, much stronger, forcing the US to interact directly with the regime in Pyongyang, allow the North to keep their nuclear weapons for fear they could detonate them within the captured Seoul, and furthermore allow the North to win greater concessions from the international community and the South, which constitutes a major victory for the North.
The danger of that happening is slim, but the risks for full scale war are there, even if no side desires to fight. The North cannot win a war, unless it is able to strike first and possibly capture Seoul, which is no easy task. Neither can it sit by and allow its infrastructure and conventional forces to fall further and further behind their enemies, reducing their effectiveness and weakening their negotiating position--thus the threat of a limited strike by Pyongyang remains on the table, in order to secure Kim's standing and possibly win some concessions from their enemies. The South, for its part, has no wish to damage economic growth and prosperity for the sake of a ruinous war which could see large parts of Seoul reduced to rubble, but neither can the government sit by and allow the North to test weapons or engage in limited strikes without severe military retribution. The United States cannot be seen to abandon its South Korean ally, and if necessary, would join with any strike back against the North, and this remains the most likely possibility for full-scale war to break out. Retribution from one side is exceeded by the other until a state of war exists.
As it is, it's impossible to know if that will happen. If it's true the North are preparing a test missile launch and another nuclear test, then it's clear Kim understands the dangers of a limited strike. The US has already toned down its rhetoric and reduced military activity, so limited concessions may be in sight for Pyongyang. However, if the new tests result in further action at the UN and more sanctions enforced by the South and the States, Kim only has one more card to play to cement his authority--the limited strike option, which invites full-scale war and the end of the regime.







