After the first two months of the conflict, South Korean forces were on the point of defeat, forced back to the Pusan Perimeter. In September 1950, an amphibious UN counter-offensive was launched at Inchon, and cut off many of the North Korean attackers. Those that escaped envelopment and capture were rapidly forced back north all the way to the border with China at the Yalu River, or into the mountainous interior. At this point, in October 1950, Chinese forces crossed the Yalu and entered the war. Chinese intervention triggered a retreat of UN forces which continued until mid-1951. After these dramatic reversals of fortune, which saw Seoul change hands four times, the last two years of conflict 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.