Warmatrix

The Korean War, lasting from 1950 CE to 1953 CE, was the first large conflict of the Cold War. It divided Korea into two halves, a situation that exists until this day. Militarily, jet aircraft showed themselves superior to older propeller-driven aircraft and helicopters were used on a large scale for the first time.
Some decades before and during World War II Korea was ruled by Japan. At the end of the war the USSR chased the Japanese out of Manchuria and northern Korea, while the USA occupied the southern half. The two superpowers dominated politics. In 1948 CE a communist government was set up in the north and a capitalist republic in the south. The North Korean leader Kim Il-Sung aimed to re-unite the peninsula under communist rule; the southern leader Syngman Rhee strove for the opposite, i.e. unification from the south. North and south fought each other in small yet bloody skirmishes. A year later the USSR gained access to nuclear weapons and Mao's communists gained the upper hand in the Chinese Civil War. Now confident, the USSR started to arm and train the North Korean army. When China's leader Mao also promised support in 1950 CE, North Korea invaded the south in late June.
The North Korean army numbered 10 infantry, 1 tank and 1 air force divisions, some 150,000 - 200,000 men in total. In contrast, the south had only 95,000 men in 8 divisions, half of them understrength, and a small air force. They had no tanks or anti-tanks guns, because the USA had been afraid they would use them to attack North Korea. Within a week after the invasion the South Korean army was reduced from 95,000 men to some 22,000 and forced to give up the capital Seoul. Many men defected to North Korea; others fled. There were 4 American divisions in the country, though because of budget reductions and emphasis on nuclear weapons, these were but a shadow of their World War II counterpart. The Americans too were quickly driven south.
The USA, fearing that Japan would be the next target, decided to intervene. As the recently established United Nations had been nominally given responsibility over Korea in 1947 CE, an international coalition against North Korea was quickly established, though the USA was clearly the driving force and senior partner: throughout the war, South Korea and the USA accounted for 90% of the southern strength. Some troops were rushed to Korea, but they were lightly armed and could not turn the tide. By August, the South Korean and American forces held on to only 10% of Korea in the southeast around the city of Pusan.
However the US Air Force quickly gained air superiority and harassed the North Koreans during daylight hours, while the navy, which neither had resistance of note, blockaded the ports. Army reinforcements kept coming in from Japan, including tanks. In August, the roles started to reverse. By early September, the North was outnumbered by some 100,000 soldiers. The Americans, making use of their sea superiority, bypassed the bulk of the enemy. Led by general Douglas McArthur they made an amphibious landing at Inchon and quickly recaptured Seoul, cutting the North Korean army in two. Not retreating quickly enough, it lost many soldiers, tanks and other equipment. The Americans, pushed by McArthur but with consent from Washington, did not stop at the 38th parallel, which had been the border between the two halves for five years. They made an attempt to fully reverse Kim Il-sung's move. In a few weeks the northern capital Pyongyang was taken, while what was left of the North Korean army was reduced to fight a guerrilla war in the mountainous terrain.
Communist China, which had received military aid from North Korea during the Chinese Civil War, had already stationed many troops at the Chinese-Korean border and clashed with the USA politically. The country was wary of a possible American advance into China itself. After hesitation for a while and coercion by the USSR, it decided to intervene. In mid October 30 Chinese divisions, some 380,000 men, infiltrated Korea by night, hidden from sight of the UN air force. They received limited support from the USSR, in the form of aircraft, weapons and supplies. The Chinese marched efficiently through the often rugged terrain, often outmanevering the mechanized American forces that were largely confined to roads, though their shortage of heavy weapons and good communications repeatedly prevented them to make rapid breakthroughs. Nonetheless numbers and determination kept up the momentum. In the rest of the year they liberated North Korea.
Now it was the turn of the Chinese to cross the 38th parallel; in January 1951 CE they captured Seoul once more for the north. This prompted McArthur to consider using nuclear weapons, but this was denied by politicians in Washington. Chinese supply lines became strained and the increasingly international UN forces started to rally, recapturing Seoul again in March. The Chinese air force joined in, challenging the UN air superiority with limited success, though they deterred the aging American B-29 bombers.
By July 1951 CE both forces were facing each other on both sides of the 38th parallel. The war on the ground descended into a stalemate, spiked by occasional offensives. The south retained its air superiority and bombed North Korea relentlessly, causing grave supply problems for the Chinese. As a result, the latter suffered many more casualties. Over the course of the war, North Korea was bombed mostly into rubble.
In July 1953 CE an armistice was signed and a demilitarized zone established along the 38th parallel. The Chinese and most UN troops withdrew. However the war was never officially ended and continues until today, though the armistice limits fighting to small border incidents. In the meanwhile North Korea has developed nuclear weapons of its own and might restart the war, though that would be a dangerous move as its relations with China and Russia have soured.
During the war, the strength of the forces involved fluctuated heavily. They peaked at about 1 million men on the southern and 1½ million on the northern side. Both sides suffered several hundred thousand casualties, most by North Korea. There were numerous war crimes and massacres, conducted by both sides. As a result, civilian casualties were also high, around 2.5 million, again mostly in the north.
The Korean War was the first miltary conflict between communist USSR and capitalist USA, with China and some western countries thrown in to boot. The Koreans themselves started it, but eventually had little influence over the outcome, though they bore the brunt of the fighting. The rolling back and forth of the frontline was the result of overestimation of strength in turn by the USA, Kim Il-sung, again the USA and finally Mao. Both the USSR and USA had nuclear weapons at their disposal, yet the war was fought by conventional arms. Officially the USSR claimed neutrality and when evidence of their participation surfaced, other parties pretended not to notice, to prevent escalation of the conflict.