Map: Chinese Spring Offensive 22 April-20 May 1951

May 16, 1951 – Chinese Communist Forces launch a second offensive in Korea and gain up to 20 miles of territory.

Map: UN Spring Offensive 22 May-8 July 1951

May 21, 1951 – The U.S. Eighth Army counterattacks to drive the Communist Chinese and North Koreans out of South Korea.

Mid-June, 1951 – UN forces advance approximately 2–6 miles north of the 38th Parallel. With the start of ceasefire negotiations underway, the UN advance stops on the KansasWyoming Line. Despite some limited attacks, this remains the frontline through 2 more years of stalemate.

July 13, 1951 – Truce talks between the UN and the communists begin at Kaesŏng. However, fighting continues for two more years. 

The defeat of the Communist advance into South Korea and the restoration of a firm defensive line roughly along the 38th Parallel decided the outcome of the war and guaranteed the future of South Korea. U.S. and UN resolve in the subsequent summer fighting brought the Chinese and North Koreans to the peace table. The American response to the shock of the Chinese intervention evolved into a firm determination to limit the objectives of the war to the continued existence of South Korea.

Source: U.S. army Center of Military History