What caused North Korea to begin to push back the UN troops in the war at the Yalu River

Chinese entrance into the Korean War caused North Korea to begin to push back the UN troops in the war at the Yalu River.

Explanation:

By the end of September 1950, the North Korean Army was almost defeated, and the occupation of the entire territory of the Korean Peninsula by US-South Korean forces was only a matter of time. Under these conditions, during the first week of October, active consultations continued between the leadership of the USSR and China. In the end, it was decided to send parts of the Chinese army to Korea.

From October 19, Chinese forces, in large crowds of about 300,000 men, began to cross the Yalu River into North Korea. No formal declaration of war was made, but the troops were so-called volunteers in North Korea's armed forces. They were so called to make it clear that there was no war between China and the United States. The huge Chinese army forced the UN troops to start retreating. By the end of 1950, North Korean and Chinese troops had regained territory until the 38th latitude. They moved on and took Seoul for the second time on January 4, 1951. They advanced yet another bit into South Korea before the Americans and their allies managed to stabilize the front.  

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