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The US has several anti-ballistic missile systems in South Korea and I think some more in Japan, so why haven't they tried to shoot down the North Korean missiles, like the one that North Korea just shot over Japan?

It seems to me that if a missile is flying towards a US ally with an unknown payload and unknown final destination, that the American military would want to shoot it down before it becomes a threat. If it is because they haven't had enough warning, then it seems like the US missile shields are total BS and wouldn't help in the event of actually needing them.

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    An missile capable of destroying the kind that overflew japan cost 10-20 million USD; and using it reveal a lot about your anti missile capabilities. It's expensive and counter productive. – Antzi Aug 30 '17 at 1:21
  • I think I've read that shooting down a missile passing over its airspace would be incompatible with older interpretations of Article 9. – Andrew Grimm Aug 30 '17 at 1:48
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    Note, if an entire country fires a single missile, the payload is known (no payload) and the final destination is known (the ocean). There was literally 0 chance that NK decided to start a surprise war by launching a single missile at Japan. If the launch of the missile was accompanied by dozens of other missiles, and literally millions of artillery shells towards Seoul, then Japan and/or the USA would have reacted. – Scott Aug 30 '17 at 6:58
  • Only touched upon in the answers is what a poor job "missile defense" systems do in actually intercepting missiles. Also something you don't want to advertise or emphasize in detail to the crazy dude with missiles. – PoloHoleSet Aug 30 '17 at 16:43
  • Just a slight note: like the one that they just shot over Japan - on first reading, I thought "they = the Americans" (instead of "they = NK") and was very bewildered. You might want to fix that. ;) – AnoE Aug 30 '17 at 18:52
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The US didn't intervene because Japan was perfectly able to intercept the missiles and chose to not do it.

https://www.theguardian.com/world/2017/aug/28/north-korea-fires-missile-japan

Japan’s J-Alert warning system advised people across a large area of northern Japan to seek shelter. Japan’s self-defence forces did not attempt to shoot down the missile and there were no reports of damage from falling debris.

The reason why Japan didn't intercept the missile was that they quickly measured its trajectory and concluded that it wouldn't hit them. I believe they preferred the missile to fall into the Pacific instead of having pieces of it raining on their cities.


EDIT: In response to a comment below doubting Japan's ability to intercept missiles, here is information about missile interception country by country, emphasis on the relevant part :

In 2016, the Japan Air Self Defense Force (JASDF), and also the Japan Ground Self Defense Force (JGSDF), initiated their 52nd year of annual live-fire missile launches at McGregor Range, New Mexico in Fort Bliss. The 2014 annual service practice of the PAC-3 Patriot missile demonstrated a 100 percent kill rate before a group which included the commanding generals of White Sands Missile Range (WSMR), and of the 32nd Army Air & Missile Defense Command (AAMDC). Every JASDF Patriot team participated in the annual exercise, which takes several months.

Since 1998, when North Korea launched a Taepodong-1 missile over northern Japan, the Japanese have been jointly developing a new surface-to-air interceptor known as the Patriot Advanced Capability 3 (PAC-3) with the US. So far tests have been successful, and there are planned 11 locations that the PAC-3 will be installed. A military spokesman[39] said that tests had been done on two sites, one of them a business park in central Tokyo, and Ichigaya – a site not far from the Imperial Palace. Along with the PAC-3, Japan has installed the US-developed Aegis ship-based anti-ballistic missile system, which was tested successfully on 18 December 2007. The missile was launched from a Japanese warship, in partnership with the US Missile Defense Agency and destroyed a mock target launched from the coast

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    Let us continue this discussion in chat. – Rekesoft Aug 30 '17 at 8:15
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    It seems a somewhat bold assertion to go from "Japan's self-defence forces did not attempt to shoot down the missile" to "Japan was perfectly able to intercept the missiles". The former doesn't seem to imply the latter at all, and the article doesn't provide any further detail. Some additional sources might be helpful. What actually are Japan's abilities in shooting down missiles? It's worth considering that even the US's relatively advanced missile defence technology is not actually very effective, so I don't think "perfectly able" is going to be a good phrase regardless. – JBentley Aug 31 '17 at 11:49
  • @JBentley If Japan wasn't able to intercept the missile, there would be no point for them or any other country involved to state that they didn't try. By the way, that your assertion that is bold. You provided no elements backing it and didn't seem to have bothered looking it up yourself either given how quickly I found relevant information. – ksjohn Aug 31 '17 at 12:58
  • @ksjohn Aegis as well as Patriot are not meant to take out ICBMs which are still midflight/in space, as was the case with the missile as it was flying over Japan. (See e.g. en.wikipedia.org/wiki/Missile_defense#Trajectory_phase) They are meant to take out warheads after they reentered the atmosphere. In this case, the ICBM was clear of Japanese territory by the time it did so. – Thierry Aug 31 '17 at 13:20
  • @Thierry That's correct. – ksjohn Aug 31 '17 at 13:25
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If a missile is on a trajectory that takes it into the sea, then shooting it down has a small benefit (you get to practice shooting down live missiles) but a big potential downside: What if you miss.

Missile defences are rarely completely reliable. This was seen when Saddam's Iraq was attacking Israel with Scud missiles. The Patriot anti-aircraft missiles were used as defence, but unless the Patriot can get in front of the incoming missile, the missile is unlikely to be completely destroyed.

A miss, or a proximity blast that doesn't destroy the missile would be a propaganda coup for N.K. It would also leave the Japanese population feeling under greater threat. If the missile isn't on a threatening trajectory, then trying to shoot it out the sky may be a risk that is not worth taking.

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    This is a very big reason that many people don't think about. You could have a defense system that will shoot down 90% of the missiles it is privately tested on, but if you miss one in a live, public situation, you now have a 0% success rate in the eyes of the world. – JPhi1618 Aug 30 '17 at 15:06
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A complement to the great answers already here:

If you reliably try to shoot down your enemy's missiles, you provide them with a reliable environment in which to test technological means to defeat your missile defense system, such as automated evasion tactics.

They may well use this to develop their next generation of missiles.

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According to Ars Technica's article appropriately titled "Why the US and Japan didn’t shoot down latest North Korean missile" (emphasis mine):

If a missile launched by North Korea was deemed not to be a threat to Japan or the US or South Korea, the best choice available to the US and allies would be to simply observe the launch and collect data. Missing a shot at a missile just passing over Japan could have far-reaching political implications, as it would suggest that anti-ballistic missile systems are incapable of protecting people in South Korea, Japan, or Guam. And if a shot hit the missile, it could further provoke North Korea into much more dangerous actions.

It also goes on to say that the Aegis system would have a very difficult time intercepting a ballistic missile over Japan to a target such as Guam.

Aegis-equipped destroyers and cruisers would have to be dangerously close to the North Korean coast to get a chance to strike an ICBM in "boost" phase as it rose and could be vulnerable to North Korean submarines if an actual attack were planned.

More local "point defense" missiles would have a better opportunity to intercept, as they'd have greater warning. But while they might have more time to intercept a missile coming in from a high trajectory at Japan (or to intercept an attack on Guam with missile batteries there), the chances of actually making an atmospheric intercept against such missiles could be substantially lower.

Again, by declining to intercept the missile, it does not definitively show one way or another if the anti-ballistic missile system are capable of shooting down such a missile.

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Once it is over Japan, shooting it down will make you responsible if anyone gets hurt - if it crashes without you attacking it, and someone gets hurt, it is the fault of who launched it. If you shoot at it to make it crash, it is your fault...

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