Operation Rugged was a military operation performed by the United Nations Command (UN) during the Korean War designed to advance the UN lines to positions north of the 38th Parallel designated the Kansas Line. The operation would be the first phase of the advance, being immediately succeeded by Operation Dauntless which would take the UN forces to the Wyoming Line to north of the 38th Parallel.
Background On 22 March, following the successful conclusion of Operation Courageous, US Eighth Army commander Lieutenant General Matthew Ridgway notified UN forces commander General Douglas MacArthur that he was developing plans for an advance that would take Eighth Army forces above the 38th Parallel to a general line following the upstream trace of the Ryesong River as far as Sibyon-ni () in the west, falling off gently southeastward to the Hwach'on Reservoir, then running east to the coast. As in past and current operations, the objective would be the destruction of enemy troops and materiel. MacArthur approved Ridgway's concept but also scheduled a visit to Korea for 24 March, when he would have an opportunity to discuss the plans in more detail.
Upon his return to Tokyo late on 24 March, following his conference with Ridgway and a visit to the front, MacArthur announced that he had directed the Eighth Army to cross the 38th Parallel "if and when its security makes it tactically advisable. More specifically MacArthur had approved Ridgway’s concept of a general advance as deep as twenty miles into North Korea.
On 27 March, Ridgway assembled Corps and Division commanders at his Yeoju headquarters and advised them that ceasefire negotiations and future US Government decisions might compel the Eighth Army to adopt a static defense. Because of its inherent rigidity, such a stance would require strong leadership and imaginative tactical thinking, he warned, to stand off a numerically stronger enemy that might not be similarly inhibited in the choice of tactics. The Eighth Army meanwhile would continue to move forward and in the next advance would cross the 38th Parallel. Ridgway agreed with MacArthur's earlier prediction that a stalemate ultimately would develop on the battlefront, but just how far the Eighth Army would drive into North 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 24 March. 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 north-east and Seoul in the south-west. 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 (ROKA) 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 counterattacks. IX Corps was to release the US 1st Cavalry Division. The division was to assemble at Kyongan-ni, below the Han River southeast of Seoul, and prepare to meet enemy attacks aimed at the capital via Route 1 from the northwest, over Routes 33 and 3 from the north, or through the Bukhan River valley from the northeast. In the X Corps' zone, the bulk of the US 2nd Infantry Division was to assemble at Hongch'on ready to counter an attack following the Route 29 axis, and a division yet to be selected from one of the two ROKA Corps in the east was to assemble at Yuch'on-ni on Route 20 and prepare to operate against enemy attacks in either corps sector. The 187th Airborne Regimental Combat Team (187th RCT), which had left the I Corps' zone for Taegu on 29 March, meanwhile was to be ready to return north to reinforce operations wherever needed.
While these forces established themselves in reserve, Ridgway planned to launch Operation Dauntless, a limited advance toward the Iron Triangle by I and IX Corps. With the objective only of menacing the triangle, not of investing it, the two Corps were to attack in succession to Lines Utah and Wyoming. They would create, in effect, a broad salient bulging above the Kansas Line between the Imjin River and Hwach'on Reservoir and reaching prominent heights commanding the Ch'orwon-Kumhwa base of the communications complex. If struck by strong enemy attacks during or after the advance, the two Corps were to return to the Kansas Line.
To maintain, and in some areas regain, contact with enemy forces, Ridgway allowed each corps to start toward the Kansas Line as it completed preparations. The Operation Rugged advance, as a result, staggered to a full start between 2 and 5 April. On 3 April, Ridgway updated MacArthur on the operational plans. MacArthur agreed with the Operation Rugged and Operation Dauntless concept, urging in particular that Ridgway make a strong effort to hold the Kansas Line. At the same time, MacArthur believed that the two operations would move the battlefront to that "point of theoretical stalemate" he had predicted in early March. Once Ridgway's forces reached their Kansas-Wyoming objectives MacArthur intended to limit UN operations to reconnaissance and combat patrols, none larger than a battalion.
Operation Ridgway had widened the I Corps' zone eastward to pass control of the US 24th Infantry Division, which had been operating on the IX Corps' left, to I Corps commanded by General Frank W. Milburn. While Milburn's forces along the Imjin River stood fast, the 24th and 25th Infantry Divisions in the eastern half of the I Corps' zone attacked north on either side of Route 3 on the morning of 3 April. East of the road, the 24th Division moved astride the Yongp'yong River valley, the 5th Infantry Regiment on the left advancing into the Kwanum Mountain mass abutting Route 3, the 21st Infantry Regiment striking for Kungmang Mountain just inside the right Corps' boundary. West of Route 3, the 27th and 35th Infantry Regiments of the 25th Division advanced toward high ground rising between a lateral stretch of the Yongp'yong River and the Hantan River farther north.
Pushing scattered People's Volunteer Army (PVA) 26th Army forces out of position by fire and occasionally by assault, and turning back a few light counterattacks, the 25th Division took the heights overlooking the Hantan River on 5 April. Resistance to the 24th Division was desultory except at the far right where the 2nd Battalion, 21st Infantry, stalled on the western slopes of Kungmang Mountain on 4 April, under fire from a strong PVA 40th Army force dug in on the crest behind barbed wire and antipersonnel mines. The battalion finally cleared the position after air strikes and artillery fire had softened it on the morning of 5 April. The 5th Infantry occupied the Kwanum Mountain mass that same day; slowed by the Kungmang battle, the 21st Infantry reached the Kansas Line on 6 April.
The adjoining British 27th Brigade following the Kap'yong River valley on the IX Corps' left was stopped by PVA fire from Kungmang Mountain until the 21st Infantry reduced that position; then the British marched unopposed to the Kansas Line on 6 April. Flushing scattered PVA out of the IX Corps' central zone, the ROKA 6th Infantry Division reached its Kansas objectives the same day. On the Corps' right, where the 1st Cavalry Division advanced astride the Pukhan River, the attached 7th Marine Regiment moved easily up the west side of the river, but the 7th and 8th Cavalry Regiments attacking through cut-up, virtually roadless ground east of the Pukhan were slowed by strong delaying forces of the PVA 39th Army. On 6 April the two cavalry regiments were still some short of their Kansas Line objectives adjacent to the Hwach'on Reservoir.
Hwach'on Reservoir
Ridgway suspected that the stiff resistance to the 1st Cavalry Division was related to enemy plans to obstruct IX Corps' movement by releasing the reservoir's water through the Hwacheon Dam and flooding the Pukhan. The water was far from its maximum level, but air observers recently had noted that the dam's eighteen sluice gates were closed. The PVA were intent on keeping the Cavalry away from the reservoir to give the water time to rise before releasing it.
As the advance got under way, the IX Corps' engineer calculated that simultaneously opening all sluice gates and penstocks when the reservoir was full would raise the Pukhan to in the vicinity of the Kansas Line and would flood much of the Chuncheon basin. Although the flooding would not be disastrous, it would temporarily disrupt lateral movement in the Corps' zone and north-south traffic on Route 17, IX Corps' main supply route; moreover, this harassment could be repeated as long as the dam remained in enemy hands. Ridgway, in light of these prospects, adjusted his plans to include the dam as an objective.
The dam stood at the northwest corner of the reservoir, its spillway slanting north into a deep, narrow gorge through which the Pukhan at that point coursed north and then turned west and south to form a horseshoe-shaped loop. The structure abutted on two narrow-ridge peninsulas, one protruding south into the reservoir on the east, the other located in the loop of the Pukhan on the west. The western peninsula, which offered the only overland approach to the dam, jutted beyond the Kansas Line at the right of IX Corps; the dam itself rested above the Kansas Line at the X Corps' left. Ridgway shifted the boundary between the two Corps eastward to put both approach and objective in the IX Corps' zone and instructed Corps commander General William M. Hoge to seize the dam. With the reservoir level well below maximum, Ridgway attached no urgency to the seizure; he adjusted the Wyoming Line to include the dam, making it an objective not of Rugged but of Operation Dauntless to follow. As a Dauntless objective, the dam's capture would fall to the 1st Marine Division, scheduled to relieve the 1st Cavalry Division after the latter reached the Kansas Line.