Korea before this occurred, could not be accurately assessed at that time. Ridgway had revised his concept for advancing above the parallel since meeting with MacArthur on March 24. He planned to point his main attack toward the centrally located road and rail complex marked out by the towns of Pyonggang in the north and Ch'orwon and Gimhwa-eup in the south. This complex, eventually named the Iron Triangle, lay above the 38th Parallel in the diagonal corridor dividing the Taebaek Mountains into northern and southern ranges and containing the major road and rail links between the port of Wonsan in the northeast and Seoul in the southwest. Other routes emanating from the triangle of towns connected with Pyongyang to the northwest and with the western and eastern halves of the present front. A unique center of communications, the complex was of obvious importance to the ability of the enemy high command to move troops and supplies within the forward areas and to coordinate operations laterally. The first phase of this advance was to occupy ground that could serve as a base both for continuing the advance toward the complex and, in view of the enemy's evident offensive preparations, for developing a defensive position. The base selected, the Kansas Line, followed the lower bank of the Imjin River in the west. From the Imjin eastward as far as the Hwach'on Reservoir the line lay to above the 38th Parallel across the approaches to the Iron Triangle. Following the lower shoreline of the reservoir, it then turned slightly north to a depth of above the parallel before falling off southeastward to the Yangyang area on the coast. In the advance to the Kansas Line, designated Operation Rugged, US I and IX Corps were to seize the segment of the line between the Imjin and the western edge of the Hwach'on Reservoir. To the east, US X Corps was to occupy the portion tracing the reservoir shore and reaching Route 24 in the Soyang River valley, and the Republic of Korea Army (ROK) III and I Corps were to take the section between Route 24 and Yangyang. In anticipation of enemy offensive operations, Ridgway planned to pull substantial forces off the line immediately after reaching the Kansas Line and prepare them for counter-attacks. IX Corps was to release the US 1st Cavalry Division. The division was to assemble at Kyongan-ni, below the Han