The Battle of Changjin Lake was the first battle of the 9th Corps abroad, and due to the hasty entry into North Korea and the insufficient preparation of logistics materials, the 9th Corps paid an extremely heavy price in the face of the cold weather that was rare in North Korea for decades. Even though the environment was extremely difficult, the morale of the volunteer soldiers was high, and they still fought many brilliant victories, and the "polar bear" regimental flag of the US army, which is now preserved in the Chinese People's Military Museum, is the best historical witness. The regiment was surrounded and annihilated by the volunteer soldiers in the battle of Xinxingli, the regiment commander was killed, and the regimental flag was once used as a steamer cloth by the soldiers of the cooking class. Although the U.S. Army was met head-on by the Volunteers in the first battle that began on October 25, MacArthur was still convinced that China would not send troops on a large scale, and ordered the U.S. 10th Army and the Eighth Army to continue to advance from the east and west to the Yalu River, in a vain attempt to end the war before Christmas. 
On the Eastern Front, although the 1st Marine Division, which was ordered to advance north along Changjin Lake, did not encounter major resistance, the Left Wing Army did not have friendly forces to respond, and it was extremely cautious and advanced slowly. The commander of the 10th Army was very dissatisfied with this, and arranged for the 31st Regiment of the 7th US Division to take over the task of the 1st Marine Division on the east bank of Changjin Lake, and ordered the 1st Marine Division to capture Wuping-ri from the west bank and advance to Jiangjie, the temporary capital of Korea, and scheduled to launch a general attack on the 27th. Coincidentally, the 9th Corps, which had already assembled at the scheduled combat position, also planned to launch an attack on the US forces in the Changjin Lake area on the 27th. The 31st Regiment is the relatively famous "Polar Bear" regiment in the US military. Founded in 1813, the regiment participated in almost all of America's foreign wars, and was also known as the "Overseas Legion" because of its long-term presence in the Philippines. During the First World War, the regiment landed from Vladivostok and entered the Siberian region, where it was forced to withdraw in 1920 after being resisted by the Red Army. Due to its service in Siberia, the regiment was called the "Polar Bear" regiment and painted the polar bear badge on the regimental flag. 
At the beginning of 1932, when the Songhu War of Resistance broke out, the "Polar Bear" regiment was sent to Shanghai to strengthen its Marine Corps in Shanghai. The mission was to protect U.S. expatriates and assets, as well as to stop the influx of refugees into the concession. During its garrison in Shanghai, the regiment held a grand procession on the streets of Shanghai, with gongs and drums in the sky, showing off its military might, and in essence, like other powers, it was a humiliating stroke in the history of poor and weak China. Before leaving Shanghai, the regiment used the $1,600 worth of silver dollars looted in Shanghai to find a set of exquisite silver commemorative bowls from Chinese craftsmen, including a large cup and 65 small bowls, called the "Shanghai Bowl", which later became the necessary equipment for the regiment's major ceremonies. 
After receiving the task of taking over the 1st Marine Division, the regiment was very scattered, and its 1st Battalion was still on guard duty on the east coast of Korea. The U.S. 7th Division temporarily organized the 31st Regimental Battle Group, including: the 3rd Battalion and 2nd Battalion of the 31st Regiment, the 1st Battalion of the 32nd Regiment, the 57th Field Artillery Battalion, the 31st Regiment Heavy Mortar Company and the Tank Company, which were unified under the command of the 31st Regiment McLean. After entering the eastern shore of Changjin Lake, the regiment was still very scattered. On the 27th, the position of the regiment was as follows: the 1st Battalion of the 32nd Regiment was located at Naedongzhi at the northernmost point of the battle line, the 3rd Battalion of the 31st Regiment and the 57th Field Artillery Battalion were located at Sing-ri, about 5 km south of Naedong-ri, and the regimental headquarters was located at Houpo, about 6 km south of Sinung-ri, and a heavy mortar company was arranged between Naedongzhi and Sinsing-ri to support the 1st and 3rd Battalions. The 2nd Battalion of the 31st Regiment was still in Hamxing, the rear, and when the Volunteers launched an attack on the night of the 27th, the regiment-level battle group was actually short of one battalion. 
Although the number of people is small, the firepower is strong, and the equipment such as tanks and heavy artillery naturally does not need to be mentioned. Each company of the U.S. military is equipped with three air liaison officers with flight experience who can direct air fire support, which is probably not possible in many countries until today. Company D of its anti-aircraft artillery battalion was armed with 4 M19 double-barreled 40mm anti-aircraft guns and 4 M16 four-barreled 125mm anti-aircraft machine guns, which were originally anti-aircraft **, but China did not have aircraft to participate in the war, so it was used in ground operations. The M19 can fire 240 shells per minute at a range of two or three miles, and the power of one shell is equivalent to that of a grenade. The M16 fires 1,800 rounds per minute, and the U.S. military describes it as scythe-cutting rice. These ** are the key to the US army's ability to resist the siege of the volunteers, who lack effective means of striking the US army's armored equipment, and can only risk using flesh and blood to blast the enemy's equipment. An American soldier named Brown recalled after the war: On the evening of the 28th, while he was operating the M19 shooting, a volunteer soldier suddenly climbed up from the side, and Brown pushed the soldier down, and a South Korean soldier next to him reacted and shot the volunteer soldier to death. And there was a blasting canister left on the top armor of the M19 that had not had time to detonate, and if he hadn't discovered it in time, this M19 and himself would have been blown into the sky. 
On the night of the 27th, the 80th Division and the 242nd Regiment of the 81st Division, commanded by Zhan Danan, deputy commander of the 27th Army and commander of the 80th Division, launched an attack on the US forces in Xinxingli, and captured several high ground outside Xinxingli and Neidongzhi that night. The 4th Company of the 239th Regiment entered the village of Xinxingli and successively eliminated the American troops in several independent houses in a row as a squad, one of which was the headquarters of the 3rd Battalion of the American Army. Riley, the commander of the battalion, was seriously wounded, but narrowly escaped and was rescued by American troops after dawn. The company also attacked the position of Company A of the US 57th Artillery Battalion before dawn, killing some American troops who were still in sleeping bags, and Company A retreated to the position of Company B before withstanding the attack of the 4th Company. However, the 4th Company itself also had 67 people, and due to insufficient troops, it could only withdraw from Xinxingli after dawn, and the captured howitzers and many supplies could not be brought out. The company was interspersed with outstanding intersperse, and after the war, it was awarded the title of "Xinxingli Combat Model Company" by the 27th Army. The U.S. forces relied on their double-barreled anti-aircraft guns and four-barreled anti-aircraft machine guns to barely repel the volunteers, and at the same time, the 2nd Battalion of the 31st Regiment also began to reinforce Houpu. 
On the 28th, the U.S. military dispatched 16 tanks and more than 30 planes to try to open up a retreat route in Xinxingli. The 242nd Regiment stationed on the 1221 Heights bravely resisted, and Ye Yongan, the deputy squad leader of the 8th squad of the 9th Company, led a demolition team to blow up 3 US tanks, and other fighters destroyed a tank, repelling the US attack. That night, the 238th and 239th regiments continued to attack the American forces in Xinxingli, but due to the large attrition of the continuous fighting for one day and the strong firepower of the American troops, the progress was slow. The 240th Regiment attacked Naedongzhi, and the 1st Battalion of the 32nd U.S. Regiment was forced to withdraw to a heavy mortar position between Naedongzhi and Sinsing-ri, and the two units converged and moved closer to the American troops in Sinsing-ri. Due to the lack of strength, the volunteers were unable to stop the enemy on the highway bridge. As they entered the rear side of Xinxingli, there were still skirmishes going on, and many recalled the scene as "a scene of destruction", with the remains of soldiers on both sides lying on the ground. 
At dawn on the 29th, McLean, the commander of the US 31st Regiment, found that there were troops fighting in the south, and he thought it was the 2nd Battalion of the 31st Regiment that had rushed to reinforce him, and wanted to step forward to stop it. But in fact, this unit was the 239th Battalion of the 4th Regiment of the Volunteer Army, and McLean was knocked out by several bullets, and the American soldiers said they saw Chinese soldiers dragging him into the bushes. A U.S. military company then searched the area where McLean went missing, but found no whereabouts. However, the veterans of the Volunteer Army recalled killing a colonel in the battle and seizing part of the battle diary, which was not captured or taken away. McLean was not alive or dead, and was listed by the U.S. military as a "missing person in action". It wasn't until 2013 that a body handed over by South Korea, whose DNA was tested to match that of McLean, was finally confirmed dead, more than 50 years after the Korean War. After continuous hard fighting and serious loss of troops from cold and starvation, the 80th Division was reduced to 6 companies, each with only more than 50 people. The 239th Regiment was reduced to 5 companies, and the 240th Regiment fared slightly better. Zhan Danan decisively decided to stop the offensive and requested that the 241st Regiment of the 81st Division, located in the area of the Battle Ridge, be sent to join the battle and strengthen the offensive force. During the day, the U.S. military airdropped supplies to Xinxingli, but due to the strong wind, most of the supplies fell to the positions of the volunteers. The U.S. forces in Houpu again tried to reinforce Xinxingli, but they were still repulsed by the 1121 Regiment on the 242nd Heights, and the Volunteers did not launch a large-scale attack that night. On the evening of the 30th, the Volunteer Army regrouped the strength of four regiments for ideological mobilization and replenishment of ammunition, and continued to attack Xinxingli. Since the newly joined 241st Regiment had not yet fought with the US troops, the two battalions did not disperse when attacking, and the seven companies were completely incapacitated. The battle continued until dawn and was still in a stalemate, but at this time the American army was also on the verge of collapse, and Zhan Danan decided to continue the offensive during the day. 
On December 1, the weather improved, and the U.S. military decided to use air support to break out. The remnants of the U.S. army used a convoy of tanks and cars to break and flee all the way south, and they were constantly blocked by volunteers on the way, and their movement was very slow. At 19:30 in the evening, he was stopped on the way by the barricades set up by the volunteers and completely collapsed. The U.S. troops fled in all directions, some of them were chased and annihilated by the volunteer soldiers, and some of them fled back to Houpu along the frozen surface of Changjin Lake. The battle lasted until the 2nd, and the leader of the cooking squad was ready to steam some steamed buns for everyone to celebrate, but there was no suitable steamer cloth, so he looked around. A soldier pulls out a piece of blue cloth captured on the battlefield from his backpack, embroidered with delicate patterns and English letters. Everyone discussed the origin of this cloth, and when the battalion commander heard about it, he ran to the cooking class to find the blue cloth in the steamer, and asked the translator to identify it, which was the regimental flag of the "polar bear" regiment. From the point of view of the Volunteer Army, the US troops were basically annihilated, a small number fled, the regimental flag was captured, and the regimental commander was killed. 
But in fact, the regiment was not completely annihilated, and the number was still retained. As mentioned earlier, the regiment originally had one battalion on the coastline to guard the coastline, and the 2nd battalion, which was incorporated into the 31st regimental battle group, also did not participate in the fighting in the Hamhung area. According to the war history data of the United States, its strength in the Xinxingli area was 3,288 people, and only more than 1,600 soldiers were left who broke through to Xiajieyuli, including 1,150 wounded, and only more than 490 people were left who could continue to fight, completely losing their combat effectiveness. Because of the extremely cold weather, the logistics of the Volunteer Army were extremely difficult, and the attrition was large. However, in the battle, in the face of the ace troops of the US army, the soldiers showed the courage of the Chinese people to resolutely overcome difficulties and defy the strong enemy, which seriously hit the morale of the US army.