 Complete the following phrase:  With the 25 June 1950 outbreak of the Korean War, the North Korean Peoples Army had invaded the Republic of Korea ROK with 90,000 well-trained and equipped troops who had easily overrun the smaller and more poorly equipped Republic of Korea Army. The United States U.S. and United Nations UN began an intervention campaign to prevent South Korea from collapsing. The U.S. troops engaged the North Koreans first at the Battle of Osan, being badly defeated on 5 July by the better-trained North Koreans. From there, the U.S. and UN saw a steady stream of defeats until they had been pushed back to the Pusan Perimeter by August. At the same time, North Korean agents began to infiltrate behind UN lines and attack military targets and cities. UN units, spread out along the Pusan Perimeter, were having a difficult time repelling these units as they were untrained in combating guerrilla warfare. North Korean special forces units like the NK 766th Independent Infantry Regiment had seen great success in defeating ROK troops, prompting Army Chief of Staff general J. Lawton Collins to order the creation of an elite force which could infiltrate through enemy lines and attack command posts, artillery, tank parks, and key communications centers or facilities. All U.S. Army Ranger units had been disbanded after World War II because they required time-consuming training, specialization, and expensive equipment. With the defeat of the NK 766th Regiment at the Battle of Pohang-dong, and the strength of U.S. infantry units in question, U.S. commanders felt recreating Ranger units was essential. In early August as the 