Korean War two years of fighting became a war of attrition, with the front line close to the 38th parallel. The war in the air, however, was never a stalemate. North Korea was subject to a massive bombing campaign. Jet fighters confronted each other in air-to-air combat for the first time in history, and Soviet pilots covertly flew in defense of their communist allies. The fighting ended on 27 July 1953, when an armistice was signed. The agreement created the Korean Demilitarized Zone to separate North and South Korea, and allowed the return of prisoners. However, no peace treaty was ever signed,
Soviet Union in the Korean War pilots flying MiG-15 jets participated in battles around the Yalu River Valley on the Chinese-Korean border in the area known as "Mig Alley" and in operations against UN "trainbusting" attacks in Northern Korea, with considerable success. The lack of a shared language between Soviet, Chinese and North Korean pilots frequently led to incidences of friendly fire as other MiG fighters were mistaken for American F-86 Sabre jets and shot down. The MiG-15 was a jet aircraft, supplied in large numbers to Chinese and North Korean forces during the war by the Soviet Union. Owing to their modern design (they were