The Second Battle of Seoul resulted in United Nations forces recapturing Seoul from the North Koreans in late September 1950 during the Korean War.
Following the UN counterattack at Inchon on 15 September, UN forces consolidated their positions south of the Han River and prepared to recapture Seoul.
Planning
Before the battle, North Korea had just one understrength Korean People's Army (KPA) division in the city, with the majority of its forces south of the capital.
Mounting indications were that the battle for Seoul would be far more severe than had been the action at Inchon and the advance to the Han. Every day KPA resistance had increased on the road to Yongdungp'o, a big industrial suburb on the south bank of the Han, southwest of Seoul. Aerial observers and fighter pilots reported large bodies of troops moving toward Seoul from the north. The KPA 18th Division, on the point of moving from Seoul to the Naktong front when the landing came at Inchon, was instead ordered to retake Inchon, and its advanced elements had engaged the 1st Marines in the vicinity of Sosa. On the 17th, KPA engineer units began mining the approaches to the Han River near Seoul. About the same time, the KPA 70th Regiment moved from Suwon to join in the battle. As they prepared to cross the Han, the Marines estimated that there might be as many as 20,000 enemy troops in Seoul to defend the city. The X Corps intelligence estimate on 19 September, however, undoubtedly expressed the opinion prevailing among American commanders, that the KPA was "capable of offering stubborn resistance in Seoul but unless substantially reinforced, he is not considered capable of making a successful defense."
Battle Crossing the Han River (20–21 September) The plan for crossing the Han River called for the 5th Marine Regiment to cross at the ferry site () northeast of Kimpo Airfield and west of Seoul. A swimming party of 14 men, mostly from the Reconnaissance Company, stepped into the Han at 20:00 on the evening of the 19th, crossed safely to the north side, and found that the crossing site was suitable for LVT's. A five-man patrol then continued up the slope of Hill 125 (), but turned back short of the crest. The swimming party gave the signal for the rest of the company to cross. When eight of nine LVTs carrying the Reconnaissance Company were in the water enemy mortar and machine gun fire suddenly struck among them. The LVTs turned around and made for the south bank. An hour later the swimming party arrived there with three wounded and one missing. Its plans disrupted, the 5th Marines now began preparing for an assault crossing of the Han after daylight.
After a heavy artillery preparation against Hill 125, I Company, 5th Marines, began the assault crossing at 06:45 on 20 September. Enemy fire from automatic weapons and small arms on Hill 125 caused rather heavy casualties in I Company, but it secured the hill by 09:40. Other elements of the 3rd Battalion, still riding LVT's, encountered little or no resistance and proceeded a mile inland to cut the Seoul-Kaesong railroad and a road at the village of Nung-dong by 08:30. Still riding in LVT's they now turned right and moved southeast along the railroad track toward Seoul. The 2nd Battalion followed the 3rd Battalion across the river at 10:00, passed through it, and continued the advance. By nightfall the 5th Marines with 12 tanks, and the 2nd Battalion, Republic of Korea Marine Corps, were across the river. Engineers had begun constructing a pontoon ferry at the crossing site.
On the morning of 21 September, the 5th Marines, after repulsing an enemy company-sized counterattack, advanced southeast astride the rail and road lines paralleling the Han River. Resistance, at first light, steadily increased. The 3rd Battalion captured and turned over to Korean Marines Hill 104 (), north of the rail line and from the crossing site, and then turned north east toward Hill 296 at the northwest edge of Seoul. In the meantime, the 1st Battalion attacked and captured a series of lower hills south of the rail and road lines (). That evening the 5th Marines faced a line of hills running generally north-south along the western edge of Seoul. At the southern end of the line, near the village of Sogang, the 1st Battalion was within of the main railroad station in Seoul.
Capture of Yongdungp'o (19–21 September) Advancing on the right (south) of the 5th Marines, the 1st Marine Regiment gradually approached Yongdungp'o. Relieved by the 32nd Infantry Regiment, 7th Infantry Division, in the early afternoon of 19 September, the 1st Battalion, 1st Marines, was ready to shift northeast to hill positions captured during the day by the 5th Marines at the west edge of Yongdungp'o. Because its transportation was late in arriving, darkness had fallen before the 1st Battalion reached its detrucking point. Company A climbed to the summit of Hill 118 () to relieve the occupying force. Later C Company joined it there. Meanwhile, elements of the 1st Battalion, 5th Marines, had departed at 21:00 from Hills 80 () and 85 () nearby, because there was a deadline set for their departure that would enable them to march the to the 5th Marines' Han River crossing site.
After having lost these hills that afternoon to the 5th Marines, the KPA counterattacked just before daylight. Their attack groups left Yongdungp'o, crossed the rice paddies and Kal-ch'on Creek, a sizable stream flowing north past the west edge of the city into the Han, and arrived on Hills 80 and 85 to find them undefended. Part of the KPA force continued on to Hill 118 where A and C (Companies of the 1st Marines repulsed them. The loss of Hills 80 and 85 at the edge of Yongdungp'o that night made it necessary for the 1st Battalion, 1st Marines, to assault them again in the morning. It recaptured Hill 85 only after intense close combat in which there were many Marine casualties.
Simultaneously with the North Korean predawn attack against Hills 80, 85, and 118, a battalion-size force led by five T-34 tanks left Yongdungp'o on the Inchon highway to counterattack the 1st Marines. It ran headlong into the Marine position before daylight and in a flaming battle in the darkness the North Koreans were all but annihilated. In this fight, Private first class Gonegan knocked out two T-34s with his 3.5-inch bazooka and was in the act of firing on a third when he was killed. Dawn disclosed 300 dead KPA soldiers on the road, in the ditches, and strewn about on the adjacent slopes.
By 09:45, the main body of the 1st Marines reached the high ground overlooking Yongdungp'o from the west side of Kal-ch'on Creek. Almond arrived there at 10:00 and in a conference with Colonel Puller, the Marine regimental commander, authorized him to shell Yongdungp'o. For the rest of the day the regiment held its place on the west side of the stream while artillery shelled Yongdungp'o and planes bombed it. The artillery barrage continued into the night.
The arc of high ground west of Yongdungp'o was nearest to the city at its northern and southern extremities. At the center, a wide expanse of rice paddies and dikes, as well as Kal-ch'on Creek, separated the hills from the city. Accordingly, the best points from which to attack seemed to be the two extremities of the arc. The 1st Battalion was on the north, closest to the Han; the 2nd Battalion, 1st Marines was on the south along the Inchon highway; the 3rd Battalion, 1st Marines was in reserve. At daybreak, 21 September, the artillery resumed its preparation against Yongdungp'o. Then at 06:30, the Marines attacked. At the northern end of the arc the 1st Battalion moved off Hills 85 and 80 onto the flat rice paddy ground and across Kal-ch'on Creek into the edge of Yongdungp'o. KPA fire caused many casualties and slowed its advance. North Koreans held the dikes at the northwest approach to the city.
The 2nd Battalion at the southern end of the arc () had even harder going. KPA mortar and artillery fire from high ground on that flank took a heavy toll in Marine casualties. By early afternoon, the 2nd Battalion had suffered 85 casualties in crossing the rice paddies bordering Kal-ch'on Creek. Here at the edge of Yongdungp'o, large parts of which were now burning, the KPA fought the battalion to a standstill. The 3rd Battalion, 1st Marines, passed through the hard-hit 2nd Battalion late in the afternoon and continued the attack under heavy artillery fire.
While the heavy battles were in progress on either flank, A Company left Hill 118 and moved behind low, masking hills to approach the dike system in the rice fields near the center of the line directly west of the main part of Yongdungp'o. The company formed an assault line behind a high dike, crossed it, and then advanced through chest-high rice to the deep mud of Kal-ch'on Creek, crossed the stream, reformed in front of another dike on the far side, and then entered the city streets undiscovered. As A Company moved through the empty heart of the town the men could hear sounds of heavy fighting on their right and left. Captain Robert Barrow, the company commander, soon found that he and his men were inside the town without contact with other friendly units. Barrow realized that he was in the enemy's rear and proceeded to exploit the situation. Soon Barrow's advanced platoon on the left saw KPA troops hurrying west ward along the concrete highway from Seoul-reinforcements for the battle in progress. Its fire surprised and either destroyed or dispersed these troops. Shortly before noon, and after passing almost through the city, Barrow stopped at its eastern side (). There he placed his men in a defensive perimeter on both sides of a -high dike upon whose crest ran a surfaced road which joined at this point with the Seoul-Inchon highway.
That afternoon the North Koreans apparently were too busy in the battle at the western edge of the city to give much attention to the unit in their rear, although small groups did make feeble efforts against it. At dusk five tanks attacked A Company, which used bazookas to knock out one and damage two of the tanks. The two undamaged tanks, with machine guns blazing and cannon booming, made five passes along the deeply dug-in infantry from the levee. Then the tanks withdrew into the city. At 21:00 a KPA infantry force attacked the 3rd Platoon at the northern end of the company perimeter. The platoon repulsed five separate attacks there before midnight. Morning disclosed more than 275 KPA dead in the vicinity of the dike and road intersection and many automatic weapons scattered about on the ground.
On 21 September two developments behind the front occurred that were to affect future tactical operations. First, the third regiment of the 1st Marine Division, the 7th Marine Regiment, arrived in Inchon harbor and began unloading. Second, command of the operation passed from Admiral Struble to General Edward Almond, who at 18:00 assumed command of the Seoul operation ashore at the X Corps' command post in Inchon. At this time there were 49,568 persons, 5,356 vehicles, and 22,222 tons of cargo ashore.
The North Koreans, after their failure during the night of 21-22 September to drive Barrow's company from its advanced position at the eastern edge of Yongdungp'o, apparently abandoned the city before daybreak. The 1st Marines occupied the city the next morning. On the left near the river they reached the destroyed railroad and highway bridges over the Han River east of Yongdungp'o ().
The KPA 87th Regiment of the 9th Division and elements of the 18th Division had defended Yongdungp'o. One battalion of the 87th Regiment reportedly suffered 80% casualties in the fighting there. Prisoners revealed that this regiment had left Kumch'on on 16 September to reinforce the Seoul area, traveling in trains that hid in tunnels during the day, and had arrived in the Yongdungp'o area on 20 September, barely in time to enter the fight there.