In 2017, the main threat that North Korea poses to South Korea (aside from the nuclear weapons) is that Seoul is within range of North Korean artillery. For instance (from an article in The Atlantic):
One high-ranking U.S. military officer who commanded forces in the Korean theater, now retired, told me he’d heard estimates that if a grid were laid across Seoul dividing it into three-square-foot blocks, these guns could, within hours, “pepper every single one.”
I believe that long-range artillery predates the Korean War. This makes me wonder, why did South Korea (and the international military forces) consider this an acceptable situation? Why would they leave the capital city exposed like that? (Was the military cost of gaining further territory too high?)