There were only two new developments in the Korean War, T. R. Fehrenbach explains (in This Kind of War), the general use of jet aircraft, and the widespread use of rotary-wing craft for evacuation, transport, and reconnaisance:
In the first days of the war, American Far East Air Force had knocked down the antiquated YAK-9 and YAK-15 fighters of North Korea. It was not until 31 October 1950 that a new phase of air warfare began.
On that date Russian-built MIG-15 jet fighters appeared in strength over North Korea. They raised havoc with the lumbering B-29′s bombing the Yalu bridges, and threw a fright into American pilots flying World War II F-51′s and Corsairs. On 8 November an American F-80 shot down the first MIG-15, but the Air Force was forced to rush its newest and best fighters, the F-86 Sabrejets, to the Far East.
[...]
The Communist aircraft, although field after field was constructed in North Korea, and as quickly bombed out, never were based south of the Yalu. They remained, silvery in plain sight on broad airdromes just north of the river, in privileged sanctuary, coming now and again across the river to engage patrolling American aircraft above the Valley of the Yalu — the famous MIG Alley.
American aircraft were never permitted to cross the Chinese or Russian boundary, even in hot pursuit.