Twenty-three years have passed since Charlie T asked for the chords. I've found the sheet music for the fiddle tune of 'Bonaparte Crossing the Rhine" at abcnotation.com   

That version claims to be in the key of G, but I am suspicious. To me the only F# in the piece sounds ugly and should probably be an F. That would put the piece in the C family.

The chords for that ought to be easy to sound out. It starts with two pick-up notes, which you can ignore. Then there's a lot of Am and G, with Am being more common. Sometimes there are C chords, usually half a measure at a time.

Measures often end with two eighth notes which don't go with the rest of the measure. You have to decide whether to just let the accompaniment die out at that point or to make a chord change. That depends on how important those notes sound to you.