I would argue that Douglas MacArthur is much more important in US history than in world history. However, he did have an impact on the history of the world as well. For the US, he is important both for the way in which he helped the US to win WWII and for the way in which his actions in the Korean War ended up challenging our very system of government-military relations.
In WWII, General MacArthur was very important because he was the leader of the American forces in the Pacific. It was under his leadership that the US conducted its “island hopping” push towards Japan. This is important both for US history and for world history because that strategy helped to defeat Japan and bring an end to WWII.
MacArthur is also important in US history because of his actions in the Korean War. In that conflict, MacArthur was leading American forces in Korea. He felt that the civilian government, under President Harry Truman, was not fighting the war aggressively enough. He publicly questioned Truman’s leadership and his strategies. He also issued statements of his own that seemed to be attempts to set policy for the US. This led to Truman relieving him of command. MacArthur had tremendous backing from Republicans in the US, but Truman persisted in removing him and had his decision upheld. This was very important to the history of the United States because it helped to reestablish the idea that military commanders are to be subordinate to civilian authorities in all ways.
Thus, MacArthur is important to US history and world history because of his importance in the Pacific theater of WWII. He is further important to US history because of his actions in Korea and the response that they elicited from President Truman.