Battle of Chosin Reservoir
The Battle of Chosin Reservoir, also known as the Chosin Reservoir Campaign or the Changjin
Changjin is a mountainous county in South Hamgyong province, North Korea.Changjin lies on the Rangrim and Pujonryong ranges, and most of the county sits atop the Kaema Plateau. Due to this location, Changjin has a particularly cold climate. The highest point is Ryonhwasan . The chief stream is...

 Lake Campaign
,Official Chinese sources refer to this battle as the Second Phase Campaign Eastern Sector (第二次战役东线). The Western Sector is the Battle of the Ch'ongch'on River
Battle of the Ch'ongch'on River
The Battle of the Ch'ongch'on River, also known as the Battle of the Ch'ongch'on or the Second Phase Campaign Western SectorThe Eastern Sector is the Battle of Chosin Reservoir. , was a decisive battle in the Korean War, and it took place from November 25 to December 2, 1950 along the Ch'ongch'on...

was a decisive battle in the Korean War
Korean War
The Korean War was a conventional war between South Korea, supported by the United Nations, and North Korea, supported by the People's Republic of China , with military material aid from the Soviet Union...

. Shortly after the People's Republic of China
People's Republic of China
China , officially the People's Republic of China , is the most populous country in the world, with over 1.3 billion citizens. Located in East Asia, the country covers approximately 9.6 million square kilometres...

 entered the conflict, the People's Volunteer Army
People's Volunteer Army
The Chinese People's Volunteer Army was the armed forces deployed by the People's Republic of China during the Korean War. Although all units in the Chinese People’s Volunteer Army belonged to the People's Liberation Army , the People's Volunteer Army was separately constituted in order to...

 9th ArmyIn Chinese military nomenclature, the term "Army" (军) means Corps
A corps is either a large formation, or an administrative grouping of troops within an armed force with a common function such as Artillery or Signals representing an arm of service...

, while the term "Army Group" (集团军) means Army
Field army
A Field Army, or Area Army, usually referred to simply as an Army, is a term used by many national military forces for a military formation superior to a corps and beneath an army group....

infiltrated the northeastern part of North Korea
North Korea
The Democratic People’s Republic of Korea , , is a country in East Asia, occupying the northern half of the Korean Peninsula. Its capital and largest city is Pyongyang. The Korean Demilitarized Zone serves as the buffer zone between North Korea and South Korea...

 and surprised the US X Corps at the Chosin Reservoir area.