North Korea is a poverty-stricken country of an estimated 25 million people, led by a basket case of a morally corrupt, bankrupt Communist monarchy, Tim Marshall explains (in Prisoners of Geography: Ten Maps That Explain Everything About the World), and supported by China, partly out of a fear of millions of refugees flooding north across the Yalu River:
The United States, anxious that a military withdrawal would send out the wrong signal and embolden North Korean adventurism, continues to station almost thirty thousand troops in South Korea, and the South, with mixed feelings about risking its prosperity, continues to do little to advance reunification.
[…]
North Korea continues to play the crazed, powerful weakling to good effect. Its foreign policy consists, essentially, of being suspicious of everyone except the Chinese, and even Beijing is not to be fully trusted despite supplying 84.12 percent of North Korea’s imports and buying 84.48 percent of its exports, according to 2014 figures by the Observatory of Economic Complexity.
[…]
To its captive population it says it is a strong, munificent, magnificent state standing up against all the odds and against the evil foreigners, calling itself the Democratic People’s Republic of Korea (DPRK). It has a unique political philosophy of Juche, which blends fierce nationalism with Communism and national self-reliance. In reality, it is the least democratic state in the world: it is not run for the people and it is not a republic. It is a dynasty shared by one family and one party. It also checks off every box in the dictatorship test: arbitrary arrest, torture, show trials, internment camps, censorship, rule of fear, corruption, and a litany of horrors on a scale without parallel in the twenty-first century. Satellite images and witness testimony suggest that at least 150,000 political prisoners are held in giant work and “reeducation” camps.
[…]
If you come from the north, then once you are over the Yalu River there are few major natural defensive lines all the way down to the sea, and if you can land from the sea, the reverse is true. The Mongols came and went, as did the Chinese Ming dynasty, the Manchurians, and the Japanese several times.
[…]
In the twentieth century the Japanese were back, annexing the whole country in 1910, and later set about destroying its culture. The Korean language was banned, as was the teaching of Korean history, and worship at Shinto shrines became compulsory.
[…]
The defeat of Japan in 1945 left Korea divided along the 38th parallel. North of it was a Communist regime overseen first by the Soviets and later by Communist China, south of the line was a pro-American dictatorship called the Republic of Korea (ROK).
[…]
The Soviets pulled their troops out of the north in 1948 and the Americans followed suit in the south in 1949. In June 1950, an emboldened North Korean military fatally underestimated America’s Cold War geopolitical strategy and crossed the 38th parallel, intent on reuniting the peninsula under one Communist government.
[…]
The North Korean leadership, and its Chinese backers, had correctly worked out that, in a strictly military sense, Korea was not vital to the United States; but what they failed to understand was that the Americans knew that if they didn’t stand up for their South Korean ally, their other allies around the world would lose confidence in them.
[…]
In September 1950, the United States, leading a United Nations force, surged into Korea, pushing the Northern troops back across the 38th parallel and then up almost to the Yalu River and the border with China.
Now it was Beijing’s turn to make a decision. It was one thing to have US forces on the peninsula, quite another when they were north of the parallel—indeed north of the mountains above Hamhung—and within striking distance of China itself. Chinese troops poured across the Yalu, and thirty-six months of fierce fighting ensued with massive casualties on all sides before they ground to a halt along the current border and agreed to a truce, but not a treaty.
[…]
The geography of the peninsula is fairly uncomplicated and a reminder of how artificial the division is between North and South. The real (broad-brush) split is west to east. The west of the peninsula is much flatter than the east and is where the majority of people live. The east has the Hamgyong mountain range in the north and lower ranges in the south.
[…]
South Korea’s capital, the megacity of Seoul, lies just thirty-five miles south of the 38th parallel and the DMZ. Almost half of South Korea’s 50 million people live in the greater Seoul region, which is home to much of its industry and financial centers, and it is all within range of North Korean artillery.
In the hills above the 148-mile-long DMZ, the North Korean military has an estimated ten thousand artillery pieces. They are well dug in, some in fortified bunkers and caves. Not all of them could reach the center of Seoul, but some could, and all are able to reach the greater Seoul region. There’s little doubt that within two or three days the combined might of the South Korean and US air forces would have destroyed many of them, but by that time Seoul would be in flames. Imagine the effect of just one salvo of shells from ten thousand artillery weapons landing in urban and semi-urban areas, then multiply it dozens of times.
[…]
The ROK’s economy is eighty times stronger than the North’s, its population is twice the size, and the combined South Korean and US armed forces would almost certainly overwhelm North Korea eventually, assuming China did not decide to join in again.
[…]
Developing the north of Korea would be building from ground zero, and the costs would hold back the economy of a united peninsula for a decade. After that, the benefits of the rich natural resources of the north, such as coal, zinc, copper, and iron, and the modernization program would be expected to kick in, but there are mixed feelings about risking the prosperity of one of the world’s most advanced nations in the meantime.
[…]
South Korea is now a vibrant, integrated member of the nations of the world, with a foreign policy to match. With open water to its west, east, and south, and with few natural resources, it has taken care to build a modern navy in the past three decades, one that is capable of getting into the Sea of Japan and the East China Sea to safeguard the ROK’s interests. Like Japan, it is dependent on foreign sources for its energy needs, and so keeps a close eye on the sea-lanes of the whole region.
[…]
In early 2015, when the Americans, South Koreans, and Japanese got down to the details of an agreement to share military intelligence they had each gathered on North Korea, Seoul said it would pass along only a limited amount of secret information to Tokyo via Washington. It will not deal directly with the Japanese. The two countries still have a territorial dispute over what South Korea calls the Dokdo (solitary) Islands and the Japanese know as the Takeshima (bamboo) islands.
I have to draw the line at “vibrant.” A country with the lowest birthrate in the world, outside monasteries and nunneries, a birthrate that is only one-third replacement, is anything but vibrant. South Korea is a cemetery.
Of course, China, and even Russia, will intervene to keep North Korea afloat. But their influence in the North is not control, and the Kims have agency, whereas the South Koreans do not. This always raises the possibility that the Kims will pull the trigger on WW III.