I am looking for border polygons from World War I (from 1914-1918). Is there a resource out there that maintains this kind of information?
This may be what you're looking for:
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euroatlas has detailled shapefiles for europe 1900, unfortunately not free, but there is a free sample, so that you can check the quality of the data. infos and download of sample: http://shop.euratlas.com/maps_gis/gis_1900.html
the IEG-maps offer good raster-maps free for personal use: e.g. europe 1914 http://www.ieg-maps.uni-mainz.de/mapsp/mappEu914Serie1.htm and http://www.ieg-maps.uni-mainz.de/mapsp/mappEu914Serie2.htm. there are also more detailled maps for diffferent parts of europe, e.g. central-europe 1914 http://www.ieg-maps.uni-mainz.de/mapsp/map914ME.htm