Hmm, that's a toughie.
I can't say that this is
exactlyl what you're looking for but it has the right themes.
We Will Remember Them... by Lilachigh
Rated: U
Summary: In 1943 Spike is captured by the British army and given a chance to save Dru's life - and his own. And in 2001, Buffy is given a chance by Quentin Travers to travel back in time to save the world - again!
Complete
I just finished reading it myself and as a History major it really caught my interest. You've got the French Resistence and France occupied by the Nazi's here. The first chapters pretty much explain the why and the how. There is also reference, if I'm not mistaken, to
Why We Fight so hopefully you'll like it.

- Arina Summer