The US Army Combat Studies Institute provides electronic copies of short books that deal with a variety of battles and campaigns. These include operations in which elements of the US Army took part as well as operations conducted by other military operations.
The US Army Combined Arms Research Library (CARL) provides access to thousands of student research papers, official reports, and journal articles about particular battles. For those wishing to conduct research in greater depth, CARL also holds provides a large number of original documents, such as operations orders and after-action reports.
The archives of Tactical Notebook, an innovative magazine that was published in the early 1990s, contain many articles about military forces in the twentieth century. These can be found in this folder.
The introduction to Between the Rivers: Combat Action in Iraq provides a short history of attempts by American soldiers to document small unit actions. (The book itself contains many descriptions of small unit actions that took place during the first two years of Operation Iraqi Freedom.)