With the sesquicentennial of the Civil War in progress, it seems like a worthwhile idea to use describing the war as an experimental project in providing overlays for the core OSM data. The intent is to use standard OSM organizational principles (nodes, ways, relations, and tags) to store the data, and device tagging schemes to represent historical concepts.
Entities & Concepts To Be Represented
In addition to depicting historic landuse (farm and forest may vary significantly from modern days), it will be necessary to represent crops, as the type of crop planted may have had substantial impact on the course of the battle, and impacts battlefield visualization today.
From Regiments to Armies. Organizations are fluid, and not always well defined, may not want to go crazy here. Simple tagging vs. relation choice. Perhaps tag using wikipedia URL as the unit tag, e.g. http://en.wikipedia.org/wiki/6th_Wisconsin_Volunteer_Infantry_Regiment and http://en.wikipedia.org/wiki/Iron_Brigade
The routes taken by Units in Campaigns and Battles. Will probably use a relation here.
Positions taken by troops, and their attitudes (direction faced, bivouac, etc.) likely to be ways for linear positions, possibly areas for bivouacs where dimensions roughly known, nodes otherwise. Linear positions will need an indication of which direction they faced (e.g., face=left or face=right, derived from direction of way) and whether they were defensive or offensive in nature.
Start times, end times, etc. A large unit (corps, division) could take hours to pass a single point, this all needs to be expressed. Likely to be represented as tags on ways, nodes, relations.
Extended sequences of movement, pursuit, and battle over time. Examples are Lee's Maryland Campaign of 1862, Grant's Vicksburg Campaign of 1863, and Grant's Overland Campaign of 1864.