Let's talk the Terminal High Altitude Area Device, or THAAD.
THAAD is basically a bunch of trucks with missiles on them that shoot at other missiles (which are first detected by radar). If you hear that, think "missile defense," and then think, "but doesn't missile defense not work?", your thought process is not shoddy.
Missile defense has a pretty mixed tracked record. Apparently Israel's "Iron Dome" works quite well, intercepting 95% of all incoming missiles. The 1976-developed U.S. Patriot system has a dismal track record of a 25% interception rate of Iraqi scud missiles fired in the 1991 Gulf War. The Patriot system has been upgraded since then, and apparently scored a "perfect nine for nine" missiles fired in the Operation Iraqi Freedom, although questions were raised about an additional 14 missiles, and, whaddya know, the Army declined to answer those questions. All of this is detailed in a nice, short, and clear article here.
All this is to say that missile defense is a bit of gamble, and furthermore, it's a very expensive gamble. Consider the relatively-expensive Iron Dome, which fires Tamir missiles. Those Tamir missiles cost fifty times the crude missiles which they shoot down. Maybe any price is worth it to defend civilians, of course, but holy cow what a price tag!
With that background, let's get to current affairs in and around the Korean Peninsula.
Plans to deploy THAAD in South Korea go back to the Obama administration, and the Trump administration has so far been enthusiastic about the planned deployment. China, however, has been less than thrilled.
The issue is that THAAD's radar will not only pick up North Korean activity; it will, of necessity, pick up Chinese activity as well, in the Manchuria region. Naturally, China, a nuclear superpower, doesn't want the United States to have the capacity to detect an early ICBM (inter-continental ballistic missile) or other launch. And that's not unreasonable; would we want China to be able to detect one of our own?
However, China doesn't have the capacity, for now, to deploy a similar radar system that would detect US missile activity. In theory, that gives us "the edge" in a potential nuclear conflict with China, although it is worth taking a deep breath and remembering that in a nuclear conflict with China, civilization as we know it will likely come to an end, so let's all play nice and hope it doesn't come to that.
China, South Korea's largest trading partner, is in a good position to retaliate against South Korea - economically - should THAAD be deployed. Some of the methods already used by China to retaliate against South Korea are detailed here. Suffice it to say, China can put the sting to South Korea pretty good. (South Korea has already lodged a complaint with the WTO about China's retaliatory behavior.)
For this reason, THAAD - ostensibly meant to defend South Korea, in large part - is not especially popular in South Korea. This Vox article does a good job running down some of South Korean opposition to THAAD, including one fellow who wrote an anti-THAAD protest message in his own blood for Pete's sake.
The current, conservative government in South Korea, which agreed to THAAD, is on its way out, due to bribery scandals and/or perhaps just plain being embarrassing (you sure can't remove a President from office in this country for that! HAR HAR!). The likely incoming President, Moon Jae-in, is not a fan of THAAD. Can he stop its deployment, though? He might not be able to assume office in time to stop the out-going, embarrassing Park Geun-he administration from deploying the system.
Meanwhile, the Republican administration of Donald Trump is, as you would expect, quite gung-ho about THAAD and generally not in a mood to cooperate with China to attempt to tame South Korea. The Trump administration doesn't show a lot of concern for what Moon Jae-in might think, and Secretary of State Rex "sure doesn't look like a strong Secretary of State so far" Tillerson has said that "all options are on the table" with regard to North Korea, i.e., that perhaps the US might consider a military strike or even invasion.
If I was the dictator of North Korea, my response to that would be "oooh I'm real scared", because honestly, the military options available to the United States all totally suck.
China is clearly getting a little sick of having to shelter and protect the nutjobs in Pyongyang, and who knows, perhaps THAAD will be a "show of strength" that will convince China to part ways with North Korea. Or, perhaps, boxed into a corner by THAAD and insulted generally by the Trump administration, China will not only not back away from pressuring North Korea but will up its game against America elsewhere, especially in the South China Sea, where it - not the United States - currently seems to hold the cards.
Why does the South China Sea matter? Oh, you know, this:
An estimated $5 trillion worth of goods are transported through South China Sea shipping lanes each year, including more than half the world’s annual merchant fleet tonnage and a third of all maritime traffic worldwide.
Oil transported through the Malacca Strait from the Indian Ocean, en route to East Asia via the South China Sea, is triple the amount that passes through the Suez Canal and fifteen times the volume that transits the Panama Canal.
In short, this author's opinion: if THAAD was truly a lock to be a working anti-missile defense system, any amount of diplomatic turmoil with China and South Korea would surely be worth it. But because it's not a lock, deploying THAAD seems like a very heavy-handed approach to take that alienates China and South Korea while providing no guarantee of security at all. Global diplomacy guideline: if the move you're making is real macho, mull it over careful ahead of time, because the odds are it's stupid and counterproductive.
Lest you think I am being excessively harsh on the Trump administration - let's bear in mind that THAAD originated during the Obama years!
I will be sure to follow up on THAAD in the future.