| It never changes|
|A view to kill|
“”If you should get kicked in the teeth, I shall not lift a finger. You have to ask Mao for all the help.
|—General Secretary Joseph Stalin to President Kim Il-Sung of the DPRK, April 1950|
Though fought on the Korean Peninsula, it was mostly a dick-swinging match between Communist China and a capitalist coalition operating under the aegis of the United Nations and directed (and principally fought) by the United States.
The Korean civil war began on the 25th of June 1950 when Chairman Kim Il-Sung's DPRK (North Korea) was permitted by Soviet Union under Joseph Stalin and helped by the People's Republic of China under Mao Zedong to wage war upon the RoK (South Korea). Each leader had conflicting priorities, though as Communist ideologues each favoured the deposition of the South Korean Capitalist Kleptocracy of Syngman Rhee on principle.
The war can be traced back to the conclusion of World War II. After successfully acquiring the Korean 'Hermit' Kingdom in the Russo-Japanese War of 1904-5, Imperial Japan annexed the peninsula in 1910 with the Japan-Korea Annexation Treaty. After WWII ended Korea was divided into North-South along the 38th North Latitude, commonly called the 38th Parallel. The north of the country was under Soviet control, and the South was under the control of the US. This system was basically the same as what happened in Germany post-WWII.
Grand Strategy: Stalin
Stalin wanted a port on the Pacific Ocean that didn't freeze over in the winter. As their own atomic arsenal by this point was still quite small, the Soviets had two primary means of deterring US aggression, one major and one minor: their ability to invade and occupy western Europe, and their ability to disrupt US intercontinental (Americas-Europe, Americas-Africa, and Americas-Asia) trade with submarine operations. Occupying western Europe would deprive the US of its greatest trading partners, and disrupting trade would impose costs more generally. Stalin's primary focus was upon maintaining the ability to defeat Anglo-Franco-American forces in Europe, but he was also interested in building up a navy to interdict US trade. Vladivostok and other bases in the Kuriles froze over in the winter, and this complicated operations from them in the winter months. Moreover, and this was very much a secondary consideration for Stalin, the use of Vladivostok as a military port prevented it from being a conduit for Soviet trade with Asian nations.
Stalin gave Kim permission to launch an invasion of the South because he knew that whatever resulted, the Soviet Union would benefit. If the USA did not intervene then Kim would defeat Rhee's febrile regime and unite Korea under his rule. Kim's united Korea would remain underdeveloped, vulnerable to US intervention from Japan, and beholden to the Soviet Union for its development and security. Accordingly the USSR could install naval bases at the deep-water all-year natural ports of Incheon (west coast) or Pusan (southeast coast). If the USA intervened, the Soviet Union also benefited. If the USA intervened and Mao did nothing, then China would have the US forces on its doorstep. It would be more desperate for Soviet technical aid, allowing Stalin to drive harder bargains in Sino-Soviet trade agreements. If the USA intervened and Mao decided to support Kim, then the Soviet Union would be in an ideal position.
Stalin knew that in the long-term, China was destined to become more powerful than the Soviet Union because it had nearly thrice his country's population and a commensurate share of vital resources such as oil and rare-earth elements. Yet China's rise could be delayed, and the Soviet Union would be strengthened, by preventing Chinese trade with the world and forcing it into trade agreements favourable to the USSR. This is precisely what a Sino-American war would achieve. Better still, if the war escalated into an existential PRC-USA struggle then the latter would become bogged down in fighting an endless war of occupation in the most populous country on earth - a sort of Vietnam times a hundred, if you will.
That said, the war could result in US-USSR conflict. To forestall that possibility Stalin forbade Soviet Air Force pilots from speaking in Russian (which didn't work since they constantly swore in Russian, Cyka Blat) or flying south of Pyeongyang, and expedited the training of Chinese and Korean pilots to take their places.
Grand Strategy: Mao
Mao lent his approval and best General Staff operational planners to Kim Il-Sung because he believed that Rhee's regime would collapse before the USA had a chance to intervene. In Mao's mind there was no possibility of the conflict escalating, since the South Korean government was even more febrile and ineffective than Chiang Kai-Shek's had been in its final months.
Mao's optimism was justified. Rhee's regime was perceived as illegitimate due to its extreme corruption, ineffectiveness, and heavy-handed repression of opposition parties and labour unions which demanded such unpopular notions as democratic elections and nonlethal working conditions. A swathe of mass demonstrations, riots, and armed rebellions including The Jeju Island Uprising of 1948-9 indicated that the vast majority of the South Korean people opposed the government and would do little or nothing to defend it. The Christian and Fascist militias led by deportees from North Korea did act together with the police and army to crack down on all of these movements with extreme sadism and brutality, but they were relatively few in number. There were good grounds for believing that the regime would collapse almost overnight if a sufficiently massive blow were delivered to it.
Mao also believed that his victory in the Chinese Civil War had shown that the USA did not care what happened on minor fronts of the Cold War, and was primarily or solely focused on the situation in Europe. This was correct. But what he did not appreciate was the extent to which "The Loss of China" to his regime had in October 1949 just, suddenly, made Asia an important Front to the United States. President Eisenhower, like JFK and LBJ after him, felt that he could not afford to "lose" any more countries to the enemy, now that the country which would someday become the world's most powerful was on "their" side. 
Grand Strategy: Kim Il-Sung, Syngman Rhee
“”There is not a single unit in the United Democratic Forces now driving the Kuomintang from Manchuria that does not have my troops in it [...] at the end of the Manchurian campaign these troops will be seasoned, trained veterans. When the Americans and the Russians withdraw, we will be able to liberate [southern] Korea immediately.
|—Ch'oe Yonggon, DPRK Defence Minister|
Kim and Rhee both wanted to rule all of Korea, but had to bide their time. They had no indigenous ammunition, arms, vehicle and vehicle parts, or fuel production. Neither had any capacity to wage war upon the other without approval from the Soviets (Kim) or Americans (Rhee). Stalin and Truman prevented Kim and Rhee from waging outright war even as the South's Christian and Fascist militias raided the North, and Northern border guards retaliated with raids of their own. This changed in 1950 when Stalin gave his approval, several hundred lend-lease trucks, 200 WWII-vintage T-34/85 tanks, and a heap of ammunition and fuel to Kim.
Grand Strategy: Dwight D. Eisenhower
Truman had not even considered reelection[note 1] amid a shitstorm about "losing China" to Communism. Eisenhower wasn't about to repeat that mistake.
North Korean soldiers stormed across the 38th parallel on the 25th of June 1950, following the Chinese General Staff's operational plan. Things went wrong for the Chinese, and right for the USSR, almost immediately.
The USSR was still officially boycotting the UN Security Council on the grounds that the PRC should be given the "Chinese" seat (which was still held by the RoC/Taiwan). The USA was able to use the threat of cutting off Marshall Aid to the UK and France, military aid to France for its war in Indochina, and development+military aid to Taiwan to bully all three other permanent members into agreeing to resolution 84, which created a U.N. taskforce to fight the DPRK. In the early stages of the war, North Korean soldiers rapidly advanced south, capturing Seoul (the capital) and forcing the South back to a relatively small pocket around the city of Pusan. The tide of the war changed when US Army General Douglas MacArthur led an amphibious assault led by US Marines at Incheon, and the UN forces managed to push the North Koreans to within a few dozen miles of the Chinese border. With one of the two railway lines supplying their position cut, they were practically forced to retreat, abandoning all of their damaged tanks and trucks (i.e. almost all of them).
Mao didn't have much to lose and had a certain amount to gain by intervening. The European empires did not permit their colonies, or their client states in the Middle East, to trade with the PRC. On the other hand, the PRC could demonstrate its commitment to the cause of pan-humanitarian liberation from White Imperialism by backing North Korea and staying the course. For instance, on the 28th of August 1955 the black teenager Emmett Till was tortured and killed for allegedly propositioning a white woman.
Over the objections of the PLA General Staff, who did not share his optimism, Mao threw the PRC's full support behind the DPRK war effort because he believed that it might yet be sufficient to totally win the war for the DPRK. It was not.
The war settled into a stalemate pretty close to the 38th parallel with South Korea holding slightly more territory north of it than the North held south of it. All that with the cost of millions of lives and crippled people. Solid work, fellas.
“”The wrong war, at the wrong place, at the wrong time, and with the wrong enemy.
|—General Omar Bradley, reflecting on MacArthur's shenanigans|
During the war, General MacArthur began announcing his plans to push into Northeast China. However, President Harry Truman did not think that it was proper to expand the war into China (citing the Security Council resolution which only gave them authorization to fight in Korea). MacArthur did not want to listen to orders, believing China was just "temporarily allied with Soviet Russia" and that Stalin wouldn't fully support them if they were invaded. The Chinese, believing that an American invasion was imminent and troubled by the influx of retreating North Korean troops and refugees, launched a surprise attack against the UN forces, something MacArthur boasted would never happen.
He was relieved of command as a result. Because of his previous military achievements and the facts that the US was actually in no position to escalate was kept secret for national security concerns, the narrative that he was "betrayed by the politicians" has been etched into American culture, despite the possibility that he could have started World War III if someone didn't try to hold him back (although people are finally starting to realize this, thankfully).
Eventually the situation stabilized along the 38th parallel and a bloody stalemate set in. In 1953, the two sides met at Panmunjom to accept a fragile ceasefire.
Accept the Bloody Ceasefire, You Idiots
By 1953, Stalin had died and after two years of trench warfare both sides' positions were so heavily fortified that restoring mobility to the battlefield was basically impossible without using NBC weapons. Tiring of the stalemated conflict, but unable to agree on actual peace conditions, all three major powers desired a ceasefire.
The USA not-so-secretly hinted that if the north didn't agree to a truce, then they would use nuclear weapons to break the stalemate and annex the north. Operational Plan (OPLAN) 8-52 to win the war mandated the use of 480 weapons immediately and a further 120 weapons as needed - half the US nuclear arsenal (1169). Eliminating every major North Korean and Chinese supply depot and railway station between the front lines and Beijing would prevent any throughput of ammunition and food from China. Consequently, Chinese frontline forces would begin dying of starvation within a month. This would enable U.N. forces to let them die or accept their surrenders, then move up to the Korean-Chinese border and hand over control of the territory to Rhee's regime. The British, Canadian, and Australian governments were privy to this plan and begged the USA not to proceed with it. Nevertheless, she persisted.
The Soviets under Khrushchev did not wish to risk a direct nuclear exchange with the USA, so when they heard of OPLAN 8-52 through their informants within NATO they decided to pressure the north into a ceasefire. If the USA implemented OPLAN 8-52, then the Soviets would be placed in an impossible situation. They would have to choose between doing nothing substantial bar moral condemnation etc, which would cause the Soviets' allies to question their loyalty and could embolden the USA to use nuclear weapons against other Soviet allies or even the Soviet Union itself. Or they could retaliate with military force, possibly causing NATO to retaliate in turn and causing the outbreak of World War Three.
Mao had also come under considerable internal pressure from within the party leadership, including such notables as Liu Shaoqi and Deng Xiaoping who were later purged for this and other 'betrayals' of Mao during the Cultural Revolution. They were wary of escalation and found the Shaoqi faction's argument that China should focus on internal reconstruction and development persuasive. The situation at the front and Soviet information on US intentions gave them the ammunition they needed to force a change in policy, and so the Chinese joined the Soviets in pressuring Kim Il-Sung into agreeing to a ceasefire.
The USA's planned assassination of Syngman Rhee was cancelled when he stopped attempting to sabotage the talks and ratified the ceasefire.
With Friends Like These
North and South Korea are quite easily two of the worst allies any country has ever had.[note 2] Both absorbed massive amounts of resources and were barely controllable.
In August-September 1956 the Chinese and Soviets attempted to overthrow Kim Il-Sung and replace him with someone less crazy (a General), but only made things worse. In response Kim purged absolutely everyone educated overseas or who had ties to foreigners (except himself, natch), abandoned Socialism, and adopted the neo-Fascist ideology of Juche which held that he was a living God to insure against any further coup attempts on his dynasty. He also blackmailed an enormous sum of what was basically hush money (reconstruction and development aid, and near-worthless trade for Korean raw materials and industrial goods) out of Mao and Khrushchev, who wanted to maintain their street cred for never betraying even their craziest allies. Khrushchev lost said cred four months later anyway, when the need to maintain the integrity of the Warsaw Pact forced him to coup Hungary.
After the Sino-Soviet split of 1959 the Kims attempted to play the Soviets off against the Chinese, but even as they engaged in border wars and the Soviets contemplated nuking Chinese nuclear research facilities to set back their nuclear program, both powers were able to consistently agree on threatening to cut off trade and aid to North Korea to keep them from going to war with the South. There is a massive backlog of "Oh Marx, what the fuck are they doing now?!" reports on China's activities in the Soviet diplomatic corps, let alone those on North Korea. But even the handful of Chinese Cold-War Era reports that have leaked out show that they, too, even under Mao, basically believed that their alliance with North Korea meant that they were chained to a foamy-mouthed violent schizophrenic. If recent wikileaks reports are any indication, that opinion hasn't changed and if anything they'd be glad to see South Korea burdened with the massive shitstorm that would ensue from attempting to integrate the North. The only problem is that the South doesn't want that, since it would tank their economy. 
Until the 1970s, South Korea was an even worse "ally" than the North. The Eisenhower Administration had to seriously consider assassinating Syngman Rhee when he was trying to prevent a truce to end the Korean War in 1953. In 1962 the Kennedy Administration had to threaten to shoot down any South Korean Air Force plane that took off without their permission, and posted patrols to threaten to shoot any Korean soldier attempting to cross the DMZ, to prevent Rhee's regime from instigating a war. During the Cuban Missile Crisis.
In addition to that, South Korea was probably one of the most unstable states in history. The famously revolution-happy French currently live in the Fifth Republic (est. 1958), having gone through four as a result of four major wars (Napoleonic, Franco-Prussian, World War Two, Algeria) since 1792; South Korea had its "First Republic" in 1945 and, with no wars since 1953, is currently onto its Sixth. Thankfully, since its successful development under State Capitalism and transition to democracy South Korea has become significantly less jingoistic. In July 2017, it became the only state with military forces on the Korean Peninsula to not threaten to rain nuclear devastation down upon its foes. If only because it doesn't have any nuclear weapons.
...yet. It could cobble one together within about six months if it really, really wanted to - as 60% of the population wanted them to do as of September 2016 (especially the over-60s, surprise surprise). This would be a spectacularly bad idea to say the least. The whole reason why Pakistan-India is so tense is that neither side has a viable second-strike capability - i.e. there is no MAD because whoever starts the war first, destroys the enemy's nuclear weapons and wins. For a given value of 'win'. If South Korea acquired nuclear weapons, both sides would have to choose between restarting the war or being annihilated.
Anti-Communist Hysteria in the English-speaking world had two triggers. The first was "The Loss of China". The second, and by far more important, was the direct fighting between Capitalist and Communist forces on the Korean peninsula. This induced the 1951 referendum on the banning of the Communist Party in Australia (rejected by 50.4% to 49.6% in favour) and the McCarthyist Second Red Scare in America, complete with Congressional Testimony by Ayn Rand on the alleged loyalties of American Citizens who, unlike her, were never citizens of the Soviet Union.
The Korean War was a success for the Soviet Union, the United States, and
South Korea Syngman Rhee. The USSR benefited from highly unequal trade with the internationally isolated PRC, Eisenhower deflected all accusations of not doing enough to combat Communism, and Rhee and his family continued to extract massive sums from the South Korean population and live in luxury. South Korea profited insofar as it kept existing, which did not seem like much of a benefit at the time, but ultimately turned out for the best by the time economic development and somewhat more democratic rule took hold.
Technically speaking, the Korean War has never actually ended. Since 1953, the peninsula has lived under a ceasefire agreement and not a proper peace treaty. That's why there are still heavily armed North Korean troops staring down heavily armed American and South Korean troops along the "Demilitarized Zone." This is tested every now and then, typically by North Koreans who take potshots at US and ROK soldiers or use their submarines to sink ROK navy ships. The DMZ is so heavily fortified that North Koreans trying to flee south have to sneak into China first if they want to make it safely.
The Korean War has been called a "police action," although very few speeding tickets were issued. In US military terms "police action" is when troops are deployed without a formal declaration of war. This is possible because the POTUS can deploy troops without having to declare war as a part of the balance of power. These actions are (normally) things like enforcing peace treaties, aiding in the enforcement of international law, and protecting US officials, civilians and territory. That it is also used as a term for entire wars that were never declared is usually ignored.
Prior to the fall of the Iron Curtain it was debated in the West whether or not Stalin actually gave the order to Kim Il Sung to commence the invasion of the South. Later records indicated that Stalin "planned, prepared, and initiated" the war after occupying North Korea and setting up its current government in 1945. The war cost nearly a million Korean and Chinese lives, and possibly 2.5 million lives in all, almost all of whom died as a result of the decision by Mao Zedong to militarily intervene in order to save North Korea from collapse.
- The ban of serving more than two terms explicitly didn't apply to him
- Well except the Vietnams, Taiwan, and Italy
- Shen, Zhihua 2000, “Sino-Soviet Relations and the Origins of the Korean War: Stalin’s Strategic Goals in the Far East”, Journal of Cold War Studies, Volume 2, no.2, p.63
- Why is Korea the "Forgotten War"?, American History
- Shen, Zhihua 2000, “Sino-Soviet Relations and the Origins of the Korean War: Stalin’s Strategic Goals in the Far East”, Journal of Cold War Studies, Volume 2, no.2, pp.44-68
- Weathersby, Kathryn 1995, “Korea 1949-50: To Attack, or Not to Attack? Stalin, Kim Il Sung, and the Prelude to War”, Cold War International History Project Bulletin, Woodrow Wilson International Center for Scholars, Washington D.C.
- Translated by Bruce Cummings and printed in Bruce Cummings' The Origins of the Korean War: The Roaring of the Cataract 1947-1950 (Princeton, 1990), p.359
- The text of UNSC 84, United Nations High Commissioner for Refugees
- The full quote on Bartelby
- You could technically argue that he was later proven correct in regards to the Sino-Soviet split, but in that case it was a matter of the Soviet Union not being hardline enough for Mao's taste (not the other way around).
- There was also rumors about the disagreement over the usage of nuclear weapons, but no documented evidence exists whether either Truman or MacArthur wanted to use nuclear weapons. They have a whole article at Wikipedia on this.
- "What are the chances for Chinese or Soviet interference?" "Very little. Had they interfered in the first or second months it would have been decisive. We are no longer fearful of their intervention." Whoops.
- Kind of like The Boss in Metal Gear Solid 3.
- Shimotomai, Nobuo, "Kim Il Sung's Balancing Act between Moscow and Beijing, 1956-1972", printed in ed. by Hasegawa, Tsuyoshi, The Cold War in East Asia (Washington D.C., 2011) pp.124-134
- Roberts, Priscilla, Introduction to H-Diplo Roundtable Review of ed. by Tsuyoshi Hasegawa, "The Cold War in Est Asia, 1945-1991", printed in the H-Diplo Roundtable Review, Volume XIII, No.30 (2012), 25 June 2012 pp. 2-11
- The President's News Conference, June 29, 1950, UC Santa Barbara
- Soviet Aims in Korea and the Origins of the Korean War, University of Minnesota, Morris
- Korea and the 38th Parallel, National Geographic