Gyeongui Line (1904–1945)
Following Japan's defeat in the Pacific War and the subsequent partition of Korea, the Gyeongui Line was divided at the 38th parallel between North and South Korea. On 25 August 1945, the Soviet Army began operating trains on the line north of Sariwon, whilst trains in the south terminated at Gaeseong. In 1946, the railways in both North and South were nationalised, creating the Korean National Railroad in the South and the Korean State Railway in the North. After the Korean War, the Gyeongui Line was split into three sections. In the South, the Seoul–Jangdan section became the KNR's Gyeongui Line, retaining the same name; in the North, the Bongdong–Pyeongyang section became the P'yŏngbu Line, whilst the Pyeongyang–Sinuiju section became the P'yŏngŭi Line.
Domino theory
Korea had also partially fallen under Soviet domination at the end of World War II, split from the south of the 38th parallel where U.S. forces subsequently moved into. By 1948, as a result of the Cold War between the Soviet Union and the U.S., Korea was split into two regions, with separate governments, each claiming to be the legitimate government of Korea, and neither side accepting the border as permanent. In 1950 fighting broke out between Communists and Republicans that soon involved troops from China (on the Communists' side), and the United States and 15 allied countries (on the Republicans' side). Though the war never officially ended, the fighting ended in 1953 with an armistice that left Korea divided into two nations, North Korea and South Korea.
Mao Zedong's decision to take on the U.S. in the Korean War was a direct attempt to confront what the Communist bloc viewed as the strongest anti-Communist power in the world, undertaken at a time when the Chinese Communist regime was still consolidating its own power after winning the Chinese Civil War.