What did MacArthur do in Korea and why did he do it?
Gen. Douglas MacArthur did many things in Korea. The two most famous are the Inchon Landing and his disagreement with Pres. Truman that got MacArthur fired.
The Inchon Landing was MacArthur's brilliant offensive move that broke the North Korean invasion. The North Koreans had pushed South Korean and US troops all the way to a very small enclave around the port of Pusan. Instead of trying to fight all the way up the peninsula, MacArthur conceived of and executed an amphibious landing at the port of Inchon near the border with the North. He did this so as to cut off the North Korean troops in the South and so as to avoid having to fight battles all the way up the peninsula.
Later on, MacArthur was fired because he publicly criticized Truman, his commander-in-chief. MacArthur wanted to be allowed to bomb North Korean supply depots inside China. He did these things because he actually wanted to instigate a war with China itself. He also criticized Truman because he had very little respect for Truman.
These are the two things for which MacArthur is most famous (or infamous) with regard to the Korean War.