The Battle of the Chosin Reservoir also known as Changjin was a battle that happened following the Inchon Landing plan and it brought the ROKA back into South Korea. It was fought through November-December 1950 and the Chinese's plan of attack was to drive the X Corps out of North Korea and destroy the 1st Marine Division.  After the Inchon Landing plan on October 7th the 1st Marine Division and the X Corps crossed the 38th parallel of latitude which was North Korea and South Korea's border. 

The X Corps led by General Edward M. Almond marched on to P'yongyang the capital of North Korea. The X Corps was made up of the 1st Marine Division led by Major General Oliver P. Smith, 7th Infantry Division captained by Major General David G. Barr, and the 3rd Infantry Division led by Major General Robert H. Soule. 

When MacArthur landed his troops he was unaware that the Chinese had been preparing since the Inchon Landing operation. The Chinese started to build troops and used them against the UNC (United Nations Command) forces. Thereafter on October 18-19 the leader of China at the time Mao Zedong, arranged CPVF (Chinese People's Volunteers Force) captained by General Peng Duhuai to attack the Eighth Army. Who at the time had spread 2 major forces beyond P'yongyang at China's border around Yalu River. Once the Chinese had prepared they led the Chinese First Offense on October 25-November 6 which crippled the Eighth Army by harming an American Division, and 4 South Korean battalions during the battle of Onjong-Unsan. 

Meanwhile on October 26 and 29 two parts of the U.S. X Corps landed on the East and a Korean battle force went to the Sino-Soviet border. The separation of these forces inclined the Chinese to lead an attack on November 2-4 to fight the South Koreans and U.S. Marines at Sudong. This attack was a failure and led to 662 Chinese casualties against the Marine force. MacArthur mistook the CPVF as weak so he commanded Almond to move his men (1st Marine Division and 7th Infantry Division) over to the Chosin Reservoir. 

At the reservoir they would move West toward a mining town named Kanggye where the Chinese and North Korea were setting up a trap to put the X Corps facing the Eighth Army. While MacArthur's plan required an 88 kilometer (55 mile) march over a road through the middle of the T'aebeak Mountains, General Smith, leader of the 7th Infantry Division wanted to repeal the plan but Almond, working under MacArthur, continued onward. The X Corps wanted to cut down North Korea's power in the form of Hagaru-ri which resided in the southern part of the reservoir that provided hydroelectric power to North Korea. 

The reservoir's Korean name was Changjin but while Japan occupied Korea in WW II the name was called Chōsen but through rough translations and rushed designing the lake was called Chosin. The actual reservoir as its name would suggest it was a desolate landscape with frigid ground only bulldozers and bombs could make trenches in. In the following days the 1st Marine Division set up camps and supply routes like the MSR (Main Supply Route) The final troops started to settle in on November 13th with the 7th and 5th Marines. Each regiment included the following: a combat team loaded with artillery, a tank army, engineers, communication with headquarters, and service crew. 

Once the heads of the 7th Marines reached Hagaru-ri they marched west of the reservoir to a place called Yudam-ni which was 22km (14 miles) away. Meanwhile the 5th Marines attentively scouted the reservoir's right back. General Smith opposed the operation so he convinced Almond to  let the marines force build at Hagraru-ri and strengthen the eastern guard with the 7th Infantry Division. Along with this Almond started commanding people to form regiments and bolster the power of existing forces like the 31st Infantry Division Regiment led by Colonel Allen D. MacLean which, totaled over 3,200 Americans and Koreans; this force was later called the Task Force MacLean. This force was used to strengthen the eastern unit during November 25th. 

Smith continued to plan out emergency supply routes and evacuations along with making groups of rifleman marines patrolling the most defenseless parts of the boundary. As the UNC (United Nations Command) strengthened the Chinese began to prepare, Peng Duhaui the Chinese Military Leader commanded the Ninth Army Group which was led by General Song Shilum that had 12 divisions in 3 different armies. This force totaled over 150,000 soldiers and primarily consisted of infantry divisions loaded with mortars and with machine guns but low on other weapons or war. 

The Ninth Army was vulnerable to UNC air strikes and they were not ready for a winter battle like the Chosin Reservoir but in spite of this Song decided the X Corps were a valuable target. Though the UNC had prepared for battle Song found an effective strategy Guerrilla Tactics like ambushes. Smith continued with caution in fear of the Chinese sneak attacks but Almond was not scared. The Ninth Army launched their attacks during the last week of November. 

By November 27-28 Task Force MacLean had disbanded from the attacks with small groups that were stuck in the bitterly cold reservoir. While on the other hand the important Marine Divisions had been able to stop or divert Chinese attacks but were crippled in the process only having 1/3-1/2 of their original firearm suppliers. America countered by countless amounts of artillery fire to assault the Chinese. 

Following this on November 29 Almond met with MacArthur in Tokyo the capital of Japan about how to save the X Corps they needed to direct them to the closest port available. The majority of the forces escaped to Hungnam and were sent over to Wonsan for their safety. Harry S. Truman still wanted to save Korea from this divide but decided brute force wasn't the right way. Plus the evacuation would save the remaining forces for a future UNC mission. 

Almond and Smith were both tasked with evacuating the forces. Almond abandoned the supplies and other resources like heavy weapons at Yudam-ni and Hagaru-ri while Smith took all of his division's resources with him and reunited with Almond at Hagaru-ri. There they took time to regroup, plan, and heal casualties. 

On December 1 the 1st Marine Division continued their march over to the evacuation; they also had received crucial resources like ammunition and fuel for the battalions. As the UNC continued their march the Chinese destroyed bridges, set up traps, and attacks against the UNC. By December 11 the final members of the rear guard had made it and started loading into the ships. 

In the aftermath the Chosin Reservoir operation was a victory for the Chinese but it was not without loss. The operation wrecked the CVPF (Chinese People's Volunteers Force) who was not able to return to battle until March of 1951. The estimates for the losses for the CVPF are roughly 40,000-80,000 casualties. The 1st Marine Division lost 4,385 soldiers in battle and lost 7,338 to the frigid temperature while the X Corps lost 6,000 American and Korean soldiers for a total of 18,173 casualties.