 Complete the following phrase:  UN units, spread out along the Pusan Perimeter, had 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 great success in defeating ROK troops, prompting U.S. 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, which had previously undertaken this role, 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 the recreation of Ranger units was essential. In early August as the 