I don't know whether this applies to french, but I learned spanish without previous knowledge by reading a whole site on one of my favorite topic - Napoleonic history
Advantages: I knew the general scenario, appreciated to learn more each time, the military vocabular is rather close from one language to another, and the descriptive of events both factual and quite repetitive in its structure - which is the best way to learn basic key terms about time, space, deeds, feelings.
Shortcomings: now I can read any spanish book or newspaper easily, but according to my son and friends, my accent and grammar in spanish are just terrible (I nearly never speak or write).
For french, a good start after some initial teaching could be for example the sum up of campaigns and battles on http://empire.histofig.com/-Batailles-.html
Another possibility is to use Wikipedia articles, toggling between french and english versions. Differences are also quite interesting.