Military records after 1922 are more difficult to access than earlier military records.
WWII had Draft cards, much like WWI had. Both sets of draft cards are available on Ancestry.com.
Records for military service is held at the National Archives.
For copies or information from the military file, write to the National Archives requesting the necessary information. They have several forms that can be downloaded filled out from their website.
For access to these records, you usually have to be the veteran or provide proof of relationship to the veteran.
Some information, such as pictures, for these various conflicts have been released and are available.
Other information on their website that may have had a general release include casualty records, records of metals, awards and decorations, enlistment and draft records, and combat operations. For more information on a particular conflict, check with the National Archives or their website.