The USA all of the sudden appears vulnerable to intercontinental missiles from North Korea. Doesn't the USA have a defence system that would easily destroy an approaching ballistic missile before it can cause any harm? Or is this whole thing only exaggerated for political reasons?
Mostly two reasons:
no defence system is infallible, for example:
- the payload of North Korea's missile could be made of several false warheads and only a few actual ones, overloading the defence system
- the defence missiles could miss their target
As this article (*) mentions
U.S. missile defenses use kinetic energy, not explosives, to destroy enemy missiles—the “kill vehicle” simply slams into the target. The kill vehicle carries no explosives, which means the kill vehicles that don’t hit the target simply continue on their merry way, re-entering the atmosphere and, for the most part, burning up. For scenarios in which the Alaska site shoots at a North Korean missile, the kill vehicles should mostly re-enter the atmosphere over Russia.
As the article continues to say, what would happen if Russia would interpret those missiles as nuclear warheads directed at them? It's not like the US can warn Russia before launching, between the detection of North Korea's launch and the launch of the defence system would pass only a few minutes, not enough to properly warn Russia about what is happening.
(*) I do not fully share the apocalyptic vision of the article, it simply happens to contain most of the necessary informations.
The threat from North Korea is not "sudden". North Korea has brazenly stated for decades that they want a nuclear missile capable of reaching the United States. They have been working to develop their nuclear weapons program since the 1990's. Their nuclear tests in 2006 and their recent missile tests demonstrate the steady and consistent progress they have made towards that goal.
While missile defense shields in general have proven effective against short- and medium-range missiles that travel through the atmosphere (the Israeli "Iron Dome" and the U.S. Patriot missle system are good examples), the United States does not have a system that can reliably protect against ICBMs. Even if it did, no missile defense shield is 100% effective, and we're talking about nukes here. It only takes one good shot to cause incredible damage and loss of life.
It is important to remember North Korea's attitude as well. Officially, the Korean War in the 1950's never ended. What the world has been living with is actually a 60-year ceasefire. North Korea and the United States are still technically at war with each other.
The western world may have largely forgotten about that war, but North Korea certainly hasn't. Anti-American propaganda is still widely distributed by their government, and the state-run media there routinely reinforces the paranoia that the U.S., along with its South Korean ally, remains poised at the border to invade at any moment. As far as they're concerned, the resumption of fighting is inevitable. The rest of the world has little reason to doubt that North Korea would absolutely use those weapons if they felt it necessary.
First, we're talking about a dictatorship that is run by a relatively young person who inherited the position - he didn't have to earn it. Also, he had his uncle executed with an anti-aircraft cannon, most probably had his own brother murdered, and spends a substantial portion of his country's meager funds on nuclear bombs and missiles to carry them. While it may not be entirely accurate to call Kim Jong Un mentally unstable, he clearly doesn't think like most global leaders.
Consequently, it is not clear if he understands just how severely the US would react if he fired a nuclear missile at them. Certainly, his bellicose rhetoric doesn't reflect that. When the leader of a nation says those things, and they have nuclear weapons, it's a very serious matter.
As for shooting the missile down... that is not guaranteed. During the cold war, both the US and USSR began to develop anti-ballistic missile systems. By the early 1970's, both sides had abandoned those efforts. The reason can be found in what were called the 'rainbow tests', very high altitude nuclear explosions that produced a rainbow display. The largest conducted by the US was Starfish Prime. Several things were learned from those tests. One was the discovery of EMP, electromagnetic pulse. The Soviets inadvertenly destroyed several of their power plants with a high altitude explosion over their own country.
One other discovery was a large cloud of radiation created in the ionosphere that lasted for several days, and was impenetrable by radar. So, all a nation had to do to disable the early ABM systems was to detonate a warhead high over the target nation, and then fire the ICBM's into that cloud.
By the time the actual warheads emerged from the radioactive cloud and could be detected by radar, they would be moving at very high speed in re-entry, around Mach 20-25, and be almost impossible to hit. That is why both nations backed away from ABM systems - too easy to defeat.
It is also why Reagan's Star Wars defense so scared the Soviets. That system was space based, and wouldn't be blinded by a high altitude nuclear explosion - it would be above the cloud and have an easy shot at nuclear missiles when they were moving at their slowest, at the top of their arc.
So if Kim could set a nuke off in the ionosphere, he could blind the radar that tracks incoming warheads. Maybe the rocket could be knocked down on it's way up - that is what THAAD is designed for. Current ABM systems are designed to knock down one or two missiles fired by a rogue state. A lot of unknowns there, and the penalty for being wrong is severe.
Finally, we have to consider our current 'news media' and it's obsession with generating profit. They have a habit of hyping anything to generate ad revenue, relevance and balance notwithstanding. In this case, it's not so much that the US government is scared of N Korea, just that the media can profit from telling US citizens that they might get nuked, even if that's not absolutely correct.
North Korea is not a threat to US, it is a treat to global monopoly and balance of power.
Currently, we have one first rate power - US. US is pretty much allowed to do whatever it wants, attack whom ever it wants without consequences on international scene.
Then we have second rate powers: China and Russia. They are allowed to somewhat oppose policies of first rate power, but with some limits. If they cross that limit - well look at sanctions aimed at Russia. These second rate powers are in constant defensive mode towards first rate power.
After that we have third rate powers, countries like India, Great Britain, Israel, France. They are allowed to have nuclear weapons, but they must align their foreign policy with policy of first rate power - US. India and France sometimes get in small diplomatic conflicts with US, but this remains mostly benign.
Rest of the countries, no matter how economically powerful they are (Japan, Germany ... ) are de facto not independent. They are usually "allies" of either US, or in some cases of China and Russia (especially if they have common borders with them) .
North Korea tries to break this pattern. Without going into discussion about NK political system, one thing is certain - North Korea tries to be independent from any country, including China. North Korea wants to get close to ideal state of autarchy, i.e. not depending on whims of any other country.
In order to that, they need to deter planet's top dog - and that is US. And to do that they need nuclear weapons. On the other hand, US simply cannot allow one small country to gain independence from it, because that could encourage others (and in fact it does - look at Iran ! ) . Situation is further complicated with Russian and Chinese interests. They too don't like nuclear NK , but they also oppose US and US drive to get right at their borders. So they reluctantly play game where they try to defuse situation, by pressuring North Korea to renounce nuclear weapons in exchange for their protection from US. As they (China and Russia) are not very successful, US is compelled to attack NK.
Look at the history of North Korea from 1950 onward, it has be one of belligerence, aggression, and violence. There are literally too many instances of type of behavior this to list here, but there are plenty of lengthy articles online detailing their exploits. by all means, go look up more if you aren't familiar with their history, they come off like some sort of bad James bond villain.
In short though, if you have a hostile state that acts in a sociopathic fashion you do not want them to be able to get access to more dangerous and destructive weapons.
Even though the OP already marked an answer accepted, it is actually exaggerated for political reasons.
Open the world map, you will learn that the ocean is huge. An ICBM that can steer a course across such a geographical area is technically difficult to build. If it was easy, the US and Russia would not use submarines carrying nuclear warheads to cruise and hide in the opposing country's nearby seas.
(update) Many people are taking the technologies for granted, as in common sense doesn't come so common. People without common sense simply through any country can copy cat a simple ballistic missile and apply it over ICBM, as in anyone can build a smart phone in their home workshop by buying parts from the electronics shop. Even if you are able to gather most part, you have no idea about the problem of supersonic speed: the sound barrier shock wave can rattle the whole payload off course. The guidance system is never easy, there is no "road" to guide the payload in the sky nor on the sea; GPS is susceptible to jamming and the Earth is NOT flat. (--update end--)
In fact, even if North Korea builds tons of them, the real threat is towards Japan, not USA, even if it is not accurate, it will become a weapon of terror similar to V2 rocket used by Nazi during World War 2. (--update--)
But the most important note is that the USA knows very well that North Korea doesn't possess the industrial capabilities to manufacture the rocket. An ICBM will disintegrate in the air quickly if all nuts and bolts are not manufactured according to a strict standard. So it is nothing but a proxy shouting game, i.e. towards China, which provides the parts and instructions.
As of 7-aug-2017 news: China agree to sanctions on North Korea. Though I don't think it is enough to ask rednecks to STFU and stop arguing that NK can build any ICBM without China helps.