The Chinese spring offensive (), also known as the Chinese Fifth Phase Offensive (), was a military operation conducted by the Chinese People's Volunteer Army (PVA) during the Korean War. Mobilizing three field armies totaling 700,000 men for the operation, the Chinese command conducted their largest offensive operation since their Second Phase Offensive in November and December 1950. The operation took place in the summer of 1951 and aimed at permanently driving the United Nations Command (UN) forces off the Korean peninsula. The offensive's first thrust fell upon the units of US I Corps and US IX Corps on 22 April but was halted at the No-Name Line north of Seoul by 30 April. On 15 May 1951, the PVA and Korean People's Army (KPA) commenced the second impulse of the spring offensive and attacked the Republic of Korea Army (ROK) and US X Corps in the east. Although initially successful, they were halted by 22 May. On 20 May, perceiving that the enemy were overextended the US Eighth Army counterattacked the exhausted PVA/KPA forces, inflicting heavy losses. Background Chinese intervention North Korea invaded South Korea on 25 June 1950. But after having conquered much of southern Korea, the KPA suffered crushing defeats after losing much of their army at the Battle of the Pusan Perimeter in early September. Outflanked by the landing at Incheon on 15 September, Eighth Army broke out of the Pusan Perimeter starting on 16 September and pursued the KPA north, in October they crossed the 38th Parallel, the dividing line between North and South Korea and invaded North Korea in turn. The Chinese government warned that to safeguard their national sovereignty, they would militarily intervene in Korea if American forces crossed the parallel. However, US president Harry Truman dismissed the warning. As the UN forces raced to the Yalu River after capturing Pyongyang on 19 October, the Chinese launched their first offensive of the war on 25 October. Undeterred, UN Commander Douglas MacArthur initiated the Home-by-Christmas offensive aimed at unifying Korea. In response, the Chinese launched their Second Phase Offensive on 25 November that forced the UN forces to retreat from North Korea in December 1950, carrying the war back south of the 38th Parallel, with Seoul being abandoned to the PVA/KPA on 4 January 1951. Reeling from these defeats, the UN Command sought to commence ceasefire negotiations with the Chinese government in January 1951,