The traditional answer given for the United States' presence is that it "deters North Korea from attacking South Korea, stabilizes the region politically, socially, and economically, and gives the United States bases from which it can project military power throughout the western Pacific." If the United States left Korea, it would somehow create a "vacuum of power," and we all know how nature abhors a vacuum (I have a problem with that argument, but more on that in a moment). My take on it is that the United States' alliance with South Korea made more sense in the aftermath of the Korean War, and even through the end of the Cold War. The problem is that we have not lived in that world for about 25 years. Even if North Korea is "the same enemy from the 1950s," they are not going to have China's backing like they did when they tried for a more conventional military attack. Plus, I don't see how removing American troops from the 38th parallel would cause World War Three. The fact that an international relations analyst over at National Interest, a Right-leaning publication in favor of a more globalist intervention with the United States army, cannot point to a case of the U.S. military deterring North Korea since before the end of the Cold War is telling to this point.
As the Cato Institute points out, South Korea outmatches North Korea on every metric of national power, except military. From 2002 to 2012, North Korea spent $4 billion annually, or 25 percent of its GDP, on military expenditures. In 2015, South Korea was spending $36.4 billion, or 9 times the amount that North Korea has been spending. What's even better is that the $36.4 billion was only 2.6 percent of South Korea's GDP. If South Korea feels that it needs to spend more money and outspend North Korea, it can afford to do so. The Global Firepower Index has South Korea ranked above North Korea. And although North Korea technically has more troops, they are underfed and lack updated technology. South Korea comes with many advantages, including a higher population, a GDP that is 40 times that of North Korea, an industrial base, and more resilient infrastructure. Even if North Korea tried to invade, South Korea would have little to no issues fending off North Korea. Let's assume that your fear is not troops crossing over the Parallel, but North Korea firing nuclear missiles into South Korea. If the leader of North Korea is already that unhinged, wouldn't it be better for South Korea to bolster its defense system (e.g., THAAD missile system) by purchasing an anti-ballistic missile system, aircraft, or some other firepower to counter North Korea's nuclear threat? And how about China exerting some pressure on North Korea so that it is less inclined to attack South Korea? How about stronger economic sanctions on North Korea?
According to a recent study from the Right-leaning American Action Forum that covers burden-sharing with allies, the United States spent $1.1 billion last year to protect South Korea, and it's easy to see how South Korea benefits by not having to cough up an extra billion dollars per annum. Granted, this $1.1 billion is only 0.1 percent of the United States' $596 billion defense budget. However, a lot could be done with a billion dollars, such as help pay off the national debt. None of this counts the creation, upkeep, or manpower required for other regional military bases created in Asia Pacific. I worry because United States debt is the highest is has been post-WWII, and has no signs of slowing down. When looking back in history, part of what brings great empires down is too vast of a military expansion. Couple that with America's burgeoning welfare system, and we have crippling debt that isn't going to help anyone. Trump was right that South Korea should be willing to pay for its "fair share" of its own defense, which quite frankly, is 100 percent. This is a point that Trump made, which happens to be similarly true with NATO. Other NATO members don't mind when the United States is footing the bill. Neither does South Korea. The United States can ill-afford to perpetuate some anachronistic, antiquated form of international welfare that provides no real benefit to United States security, and does not help South Korea truly develop its own defense and national security. I wish that Trump didn't change his mind on this one, but maybe he'll go back to his initial proposal by the time he is inaugurated.