Resource Guides: GEN: Great U.S. History Websites: 20th Cent.

Spanish-American War

War with Spain
Provides reference sources, primary sources, images and art work.
Theodore Roosevelt’s Rough Riders: Topics in Chronicling America (LOC)
Contemporary newspaper articles on the Spanish-American War.
A Guide to the Spanish American War (LOC)
Provides overview, digital primary and secondary sources and external websites

Korean War, 1950-1953
250+ primary source documents on the Korean War, obtained largely from Russian archives.From the Woodrow Wilson International Center for Scholars
more...less...
In addition to this collection, look at the broader Wilson Center Digital Archive for other specific collections on captured North Korean documents, China's involvement in the Korean War, conversations with 1948-1994 North Korean leader Kim Il Sung, inter-Korean relations, Korean War origins and armistice, and much more.
Korean War Project
The site is a bit overwhelming, but is full of quality information (primary and secondary). Includes KIA-MIA-POW-WIA databases; maps; hundreds of US Marine Corps Command and Unit files; and more.

Virtual Vietnam Archive
Largest online source for information on the Vietnam War. Browse primary sources by type. Also offers subject guides (look under the Researching menu) of popular topics.
Vietnam War (National Archives)
Provides overview, primary source material (documents, records, images and film) and links to external sources. Organized by topic.
The CIA's Vietnam Histories (George Washington U.)
Sampling of primary documents and analysis showing CIA's involvement in every aspect of the Indochina War.
Racial, Ethnic, and Religious Minorities in the Vietnam War: A Resource Guide (LOC)
Overview, primary sources and external links.