The North Korean government has threatened a nuclear attack against the United States. The threat has generally been dismissed as preposterous, and I hope it is, but I wonder. It's unlikely that they could reach the United States with a nuclear tipped ICBM, but I think that it's quite probable that they could bomb Seoul, and possibly reach Japan with some sort of nuclear warhead.
In the back of my mind are two events that took place in 2010. One was an apparent missile test that took place 35 miles off the coast of Los Angeles. A few days later, another missile contrail was seen in the Gulf of Mexico. The contrail off the California coast was dismissed as a jet, and the one over the Gulf of Mexico was ignored. At the time, the US was allowing the Japanese to test a missile in Arizona, and both China and North Korea had reason to engage in a little covert belligerence.
There is no evidence that North Korea possesses a submarine capable of reaching American waters. The longest range vessel they possess is the Sang-O model, which has a range of 1,500 nautical miles. But there is more than one way to deliver a weapon. A short range missile could be loaded aboard a freighter, for example. If it was launched within a few hundred miles of American shores, whether or not it could be intercepted and destroyed in time is problematic.
They could certainly attack Seoul, possibly a Japanese city, maybe the US, with a asymmetric delivery system of some kind.
I don't think that the threat should be dismissed out of hand. North Korea has never made such an explicit threat before. It is to be hoped that it amounted to nothing more than the meaningless bluster of a untutored young man. But calling it preposterous is a mistake. Even if it cannot be fulfilled, it is a very clear statement that Kim Jong Un should be considered dangerous, and the more so as time goes on and his country's capabilities increase.