Text Creation Partnership available
For those who like to poke around in early modern holes and corners, the digital texts of about 25,000 English publications from the late 15C up to c. 1800 are now freely available. The collection is reported to include works from Early English Books Online, Eighteenth Century Collections Online, and Evans Early American Imprints. I think there are one or two works on military history in there somewhere, if you’re willing to look.
Currently hosted at the Oxford Text Archive.