US Army Center
U.s. Army Center of Military History: Chapter Ii: The House Divided
This resource provides information on the period in Korea just before the Korean War, and on the partition of Korea by the United States and the Soviet Union.
Internet History Sourcebooks Project
Fordham University: Modern History Sourcebook: On American Intervention in Korea, 1950
The Modern History Sourcebook offers a very interesting primary source that presents the Soviet position of the beginning of the Korean War. A great resource for comparing US/USSR claims about the origins of the Korean War.