Until a few years ago, the military power equation for North Korea was that if North Korea poured its million man army into South Korea, the joint US-South Korea military had to delay the conquest of the entire peninsula long enough for the American reinforcement from Japan.

North Korea's nuclear weaponry and intercontinental ballistic missiles changed that calculation, whereby North Korea can now invade South Korea, point its nuclear missiles at Washington D.C. and/or other American cities, and tell the US president that if US reinforcements are dispatched, American cities will be nuked.

Why not bomb North Korea preemptively?

Even if its nuclear arsenal could be bombed, North Korea has 11,000 artillery pieces aimed at Seoul, which lies only 26 miles from the border. Even if each piece fired only once before being bombed, 11,000 artillery shells will rain down on Seoul. A 10 minute barrage would reduce the South Korean capital to rubble.

Why was Seoul located so close to the North Korean border?

When Japan surrendered in 1945, the American army was nowhere near the Korean peninsula but the Soviet army was close by, and poured 120,000 troops into the peninsula.

The US administration panicked, and during a discussion at the White House, sent two US army colonels into a room with nothing but a National Geographic map, and told them to come up with a demarcation line that the US government could tell the Soviets to not cross.

Not finding any major rivers or other natural boundaries, the two colonels came back with the 38th parallel, for no reason other than that it ran about halfway across the peninsula. The US President Harry Truman accepted their recommendation, as did the Soviets, and the Korean peninsula has been divided more or less along the 38th parallel ever since.

One of the two colonels was Dean Rusk, the future US Secretary of State from 1961 to 1969. Dean Rusk later admitted that he had been totally unqualified and unequipped to recommend a demarcation line. But how an army colonel advised a border that put within the range of enemy artillery the capital of the country that his army would have to help defend is regrettable at best. In any case, there is no military solution to North Korea that would spare Seoul, a city of 10 million.