Douglas MacArthur really only had one accomplishment in World War II. Of course, it was a very important accomplishment. He was the commander of the Allied forces in the Pacific as those forces fought the Japanese. Thus, we can say that he was one of the architects of the Allied victory over Japan in WWII.
At the beginning of WWII, MacArthur was commanding US forces in the Philippines. The Philippines were invaded by Japan and quickly conquered. MacArthur was ordered to escape from the Philippines and go to Australia. Once there, he was appointed the Supreme Commander of the Allied Forces in the South West Pacific Area. In that capacity, he oversaw the Allied push up through New Guinea and towards the Philippines. Later, he was appointed the commander of all of the US Army forces (which included the Air Force at that point) in the Pacific. He would have been in command of the Allied invasion of Japan if that had happened.
So, MacArthur’s major accomplishment was to be one of the main architects of the Allied victory in the Pacific in WWII.