Why the US might not shoot down a North Korean missile after all

Days after telling "Meet the Press" that the US military would shoot down any North Korean missiles headed for American territory or that of an ally, Defense Secretary Ash Carter has amended his statement. 

"If the missile is threatening, it will be intercepted. If it's not threatening, we won't necessarily do so," Secretary Carter said in his final news briefing Tuesday, as reported by Reuters. "Because it may be more to our advantage to, first of all, save our interceptor inventory, and, second, to gather intelligence from the flight, rather than [intercept the missle] when it's not threatening." 

The clarification comes amid escalating tensions between the United States and the North and rising concerns over North Korea's missile and nuclear weapons development. In his annual New Year's address on Jan. 1, Kim Jong-Un announced that the North had reached the "final stages" of intercontinental ballistic missile (ICBM) development. Two days later, US President-elect Donald Trump responded by tweeting that a North Korean nuclear weapon capable of reaching the United States "won't happen!" 

On Sunday, the same day that Carter appeared on "Meet the Press," an unnamed North Korean foreign ministry spokesman reportedly told the official KCNA news agency that North Korea can test an intercontinental ballistic missile at any time, from any location "determined by the supreme headquarters of the [Democratic People's Republic of Korea]." 

Carter has described the North's missile and nuclear weapons development as a "serious threat." On Thursday, the US said that North Korea had shown a "qualitative" improvement in its nuclear and missile capabilities following unprecedented levels of testing last year. 

Despite its claims and development progress, there is some disagreement among experts as to whether a North Korean missile could actually reach the US or Europe. North Korea has never successfully test-fired long-range missiles capable of carrying nuclear warheads, though analysts say it may be capable of doing so within the next five years. 

But the US could "learn a lot" from any non-threatening missile test, Tal Inbar, a North Korea expert at the Fisher Institute for Air and Space Strategic Studies in Israel, told CNN.

"We can analyze the trajectory and conclude some insight about the power of the engines and the amount of fuel, and estimate the potential range of the missile," he said, adding that if it is possible to retrieve the missile or rocket from the sea, there is "a wealth of intelligence in such debris."

There's some doubt, too, whether the US would be successful in intercepting a North Korean missile. A 2016 assessment released Tuesday by the Pentagon's weapons testing office found that American ground-based interceptors meant to shoot down incoming ICBMs still had low reliability, giving the system a limited capability of shielding the US from missile threats.