The Korean Peninsula is 856 kilometres long, bordered by the Sea of Japan* in the east, the Korea Strait in the south and the Yellow Sea to the west. Mainland Japan lies 160 kilometres across the Korea Strait...
The northern border with China runs along the Yalu River. In the north east there is an 18-kilometre border with Russia (then the Soviet Union) through which Soviet supplies came to North Korea. Korea has 9200 kilometres of mostly inhospitable coastline and more than 3500 offshore islands. The waist of the peninsula, where most of the war was fought, is generally 350 kilometres in width and at its narrowest 200 kilometres.
The Taebaek Range, with heights up to 1708 metres, runs down the east side of the peninsula and smaller ranges head west from it, making a barrier to the movement of armies along the central spine of the country. In 1950, road and rail lines were few and tended to run north-south along both coasts with few east-west connections. On the east coast the close proximity of mountains to the sea broke up the coastal plane. The mountains slope down more gently to the west coast, where there are wide plains and broad shallow tidal estuaries. Most of the major rivers flow from north-east to south-west, forming a further series of obstacles to north-south movement, especially within the mountains where road and rail bridges were rare.
Seoul, the South Korean capital, was an important political objective for both sides. It was also of great practical importance as the focal point of all west coast communications. Ports were few. Fortunately for the UN, Korea's greatest port Pusan (Busan) was in the south-east corner of the country, close to Japan, the UN's base for supply. Pusan was never taken by the communists.
There are extreme variations in climate in Korea. In summer, it is rainy, hot and humid. Winter brings bitter winds to Korea from Siberia and Mongolia. The winter temperature in the mountains can be extremely cold - minus 17 celsius is normal and snow covers the mountains throughout winter.
As the North Korean and Chinese armies were lightly equipped and less motorised they found it easier to operate in the mountains than the UN forces who depended upon road and rail for movement and supply. Thus in the first year of the war – the war of movement – the UN tended to launch its attacks up the coasts where, as the UN had complete control of the sea lanes, it could also use sea transport and supply. This left a gap in the intervening mountains which the communists exploited in their first counteroffensive from October 1950. In the second static phase of the war the communists continued to launch offensives in the mountains where, owing to a poor road network often closed by snow in winter, UN forces were not well able to reinforce their troops once the axis of communist attack was revealed.
* The Australian Government and the Department of Veterans Affairs do not take a position on whether the body of water between Japan and the Republic of Korea should be referred to as the “Sea of Japan” or the "East Sea".