The Command and General Staff College at Fort Leavenworth is the US Army’s school for future General officers and has, as part of its Combined Arms Research Library (CARL), an enormous online digital library that is freely accessable. This amazing online resource contains books, military field manuals and student papers covering many diverse topics. Although the focus tends to be on the US Army, there are also many works covering other periods and places and often these take the form of student papers and Masters of Military Science Thesis. One of the most useful resources for the historian are the obsolete Field Manuals from various conflicts from World War I (from how to use grenades in trenches to standard rifle drill) through to the Cold War (including Staff Officer manuals and the amazingly detailed three part series on the Soviet Army).
The digital library itself can be found at http://www-cgsc.army.mil/carl/contentdm/home.htm and is organised into the following areas:
School of Advanced Military Studies Monographs
Most SAMS students complete the regular CGSC course, then stay for a second academic year. They write either one or two monographs and are awarded a Master of Military Art and Science (MMAS) graduate degree.
Master of Military Art and Science Theses
CGSC regular course students may elect to take a set of electives, write a thesis, and earn an accredited Masters. This collection contains all of the publicly releasable theses since 1964. MMAS theses provide in-depth research on historical events, operational issues, and organizations, both existing and proposed.
Student Papers 1930-1936
Similar to the monographs above, many of those who wrote these papers would go onto command during World War II.
CGSC Student Papers
These papers are post-World War II and focus on lessons learned during that conflict.
Fort Leavenworth History
Speaks for itself.
Obsolete Military Manuals
This area is a historian’s wet dream. Military manuals from World War I to the present day are available and provide a detailed look at the way operations were conducted at the time.
World War II Operational Documents
Planning and after-action reports of the major actions fought by US troops during World War II. These documents are invaluable for those researching the how and why and when of these battles.
Military History 1900-1939
General military history with a primary focus on the United States
General Military History
General military history from World War II onwards with a primary focus on the United States
Operational Leadership Experiences
A recently started program where interviews are conducted with officers following their return from active service. A good source for understanding contemporary military issues.
Military Review
Copies of Military Review with articles covering many topics of contemporary interest at the time of publication.
CSI Press
Further papers on various topics of US military interest
Additionally, CARL provides an update page of new additions to the library. Updates tend to be fortnightly to monthly and cover all the above categories.
