[a] The Second Phase Offensive (25 November – 24 December 1950) or Second Phase Campaign (Chinese: 第二次战役; pinyin: Dìèr Cì Zhànyì) of the Korean War was an offensive by the Chinese People's Volunteer Army (PVA) against United Nations Command (U.S./UN) forces, most of which were soldiers of South Korea and the United States.

On 23 December 1950 Lieutenant General Walton Walker died in a vehicle accident in Seoul while in command of the U.S. 8th Army.

U.S. intelligence and air reconnaissance failed to detect the large numbers of Chinese soldiers present in North Korea.

The Home-by-Christmas offensive, with the objective of conquering all of North Korea and ending the war, was quickly abandoned in light of the massive Chinese assault.

[10][2]: 23  A U.S. military intelligence report dated 25 November estimated that the total number of Chinese soldiers in North Korea was only 70,000.

"[12] On 19 October, tens of thousands of Chinese soldiers began crossing the Yalu River, the border between China and North Korea, at night to avoid detection by UN reconnaissance aircraft.

On 25 October the first shots fired between the PVA and UN forces were at the Battle of Onjong, about 85 kilometres (53 mi) south of the Yalu.

Peng Dehuai, the Chinese commander, ended the First Phase Offensive because his soldiers were tired and lacked food and munitions.

He also said the Chinese soldiers were fearful of UN air raids and that the cold winter weather would make it "increasingly difficult to preserve the strength of our troops who have to sleep outside and sometimes in the snow.

Peng reasoned that if the PVA forces could destroy two or three UN divisions, the character of the war would change to the benefit of China and North Korea.

After the First Phase Offensive ended on 6 November, Peng ordered the PVA to retreat, feigning weakness and intimidation.

To induce over-confidence by the UN forces, on 18 November Peng released 103 prisoners of war (POW) (including 30 Americans) whom he had ensured would take back to the UN the false information that the PVA were suffering from shortages of food and ammunition and withdrawing to China.

"As a main objective, one of the units must fight its way rapidly around the enemy and cut off their rear....Route of attack must avoid highways and flat terrain in order to keep tanks and artillery from hindering the attack operations...Night warfare in mountainous terrain must have a definite plan and liaison between platoon commands.

"[3]: 720 To get to the battlefield, PVA soldiers walked from Manchuria to North Korea, evading detection by UN reconnaissance aircraft.

[2]: 46–47 On 24 November 1950, the Eighth Army occupied a line about 110 kilometres (68 mi) long stretching west to east from the Yellow Sea across the broad coastal plain of the Ch'ongch'on River to the ridges of the Taebaek Mountains.

[2]: 26–27 Although less numerous than the PVA who may have numbered as many as 240,000 soldiers, the 130,000[1]: 46  front-line soldiers of the Eighth Army "with modern communications, ample artillery support, tanks and backed by adequate logistic support" were "not significantly inferior in terms of combat power to the much more numerous but primitive Chinese."

[1]: 46  The U.S. Army and ROK divisions were primarily blocking roads and advancing with little resistance in northeastern North Korea and protecting the important supply ports of Wonsan and Hungnam.

[2]: 95  The UN plan was for the Marines to push westward from Chosin Reservoir across the mountains to hook up with the advancing Eighth Army and close the gap between the two components of the UN offensive.

Smith, in particular, had resisted the exhortations by X Corps commander General Edward Almond to move forward more quickly.

On the west, Walker was also warning his troops of the dangers of advancing carelessly against what seemed to many like an enemy of inferior numbers in a defensive posture.

[10]: 402 The Chinese recognized that the ROK divisions on the right (eastern) flank of the UN line were the most vulnerable units and occupied difficult, mountainous terrain.

By then, the Eighth Army had retreated to near the border of South and North Korea in what the soldiers called the "big bugout.

However, two-thirds of the 3,200 soldiers of Task Force Faith on the eastern side of Chosin Reservoir were killed, wounded, or captured.

All of North Korea is frigid in winter, but the cold was most intense for both the Chinese and the UN soldiers in the highlands around Chosin Reservoir.

By contrast the December average low for Hungnam 54 kilometres (34 mi) south at near sea level in elevation is a relatively balmy −5 °C (23 °F).

[17][18] On 28 November 1950, shortly after the onset of the Chinese Second Phase Offensive, MacArthur cabled the U.S. government in Washington that, "We face an entirely new war.

MacArthur was told "to defend...inflicting such damage to hostile forces in Korea as is possible, subject to the primary consideration of the safety of your troops.