At 0400 on 25 June 1950 130,000 North Korean (KPA) soldiers supported by 200 tanks, SU76 assault guns and 600 artillery guns, crossed the 38th parallel into South Korea...
The KPA's objective was to destroy the South Korean (ROK) army, capture Seoul and 'liberate' South Korea. In response to the invasion, some ROK units such as the 17th Regiment, the 6th Division and the 8th Division put up a determined resistance. A sea-borne KPA commando raid on the port of Pusan was destroyed. In contrast, the ROK units defending Seoul were squandered in futile piecemeal attacks against much stronger KPA forces, many ROK units losing over half of their strength.
On 27 June Seoul was evacuated amidst scenes of chaos, and soon thereafter the first KPA units entered the city. Despite at times fanatical resistance from improvised ROK units, the KPA offensive could not be halted. Very quickly UN airpower (including 77 Squadron RAAF) intervened, although it would take time to develop its full potential. Desperate to support the beleaguered South Koreans the US scratched together a force from its garrison soldiers in Japan, Okinawa and Hawaii. The first of these to engage the KPA was a 540-man battle group called Task Force Smith which, on 5 July at Osan south of Seoul, was driven back in confusion, suffering heavy losses.
While ROK units continued to retreat and resist, more US troops were sent into combat. At Kum River on 14-16 July, US 34th Regiment, which had already suffered one defeat, was forced into disorderly retreat and the US 19th Regiment was shattered. At Taejon between 19-21 July the under-strength US 24th Division was also routed by KPA attacks. The next reinforcement to arrive through Pusan, the KPA objective, was US 1st Cavalry Division which was also unable to halt the KPA.
On 25 July a battalion of US 25th Regiment was destroyed in an ambush. Elsewhere, US and ROK forces did better. The US 27th Regiment, inflicted heavy losses on the KPA, and ROK 23rd Regiment, with UN air and naval support, defeated the 5th KPA Division assault on Pusan from the north. These successful actions managed to hold firm the UN defence line. Four weeks after the war began, Pusan, in the south-eastern corner of the country, remained the only part of South Korea which had not fallen to the communists.