A map showing successive North Korean advance. The Pusan Perimeter is the border of the green portion of the peninsula.
Outbreak of war
Following the outbreak of the Korean War, the United Nations decided to commit troops in support of the Republic of Korea (South Korea), which had been invaded by the neighboring Democratic People's Republic of Korea (North Korea). The United States subsequently sent ground forces to the Korean peninsula with the goal of fighting back the North Korean invasion and to prevent South Korea from collapsing. However, US forces in the Far East had been steadily decreasing since the end of World War II, five years earlier, and at the time the closest forces were the 24th Infantry Division of the Eighth United States Army, which was headquartered in Japan. The division was understrength, and most of its equipment was antiquated due to reductions in military spending. Regardless, the 24th Infantry Division was ordered into South Korea.
In the east, the North Korean army, 89,000 men strong, had advanced into South Korea in six columns, catching the Republic of Korea Army by surprise and completely routing it. The smaller South Korean army suffered from widespread lack of organization and equipment, and was unprepared for war. Numerically superior, North Korean forces destroyed isolated resistance from the 38,000 South Korean soldiers on the front before moving steadily south. Most of South Korea's forces retreated in the face of the advance. By June 28, the North Koreans had captured South Korea's capital of Seoul, forcing the government and its shattered forces to retreat further south. Though it was steadily pushed back, South Korean forces increased their resistance further south, hoping to delay North Korean units as much as possible. North and South Korean units sparred for control of several cities, inflicting heavy casualties on one another. The ROK Army defended Yongdok fiercely before being forced back, and managed to repel North Korean forces in the Battle of Andong.
In the west, outnumbered and under-equipped US forces—committed in piecemeal fashion as rapidly as they could be deployed—were repeatedly defeated and pushed south. The 24th Division, the first US division committed, took heavy losses in the battle for Taejon in mid-July, which they were driven from after heavy fighting. Elements of the 3rd Battalion, 29th Infantry Regiment, newly arrived in the country, were wiped out at Hadong in a coordinated ambush by North Korean forces on July 27, leaving open a pass to the Pusan area. Soon after, Chinju to the west was taken, pushing back the 19th Infantry Regiment and leaving open routes to Pusan. US units were subsequently able to defeat and push back the North Koreans on the flank in the Battle of the Notch on August 2. Suffering mounting losses, the North Korean force on the west flank withdrew for several days to re-equip and receive reinforcements. This granted both sides several days of reprieve to prepare for the attack on the Pusan Perimeter.