Slitherine

Fighting this battle is somewhat tricky. I manage to seize the wagon and shortly after that the French cavalry routes from the field but my footmen continue to engage the enemy and I am told the objective fails. Should I let all the French leave the field for victory?

It's been a while since I played this, but had a brief look at the script. There is no scripted event there that would fail or complete the mission. You can rout the cavalry by taking the baggage train (wagon) but it will not affect victory conditions. That means you need to kill/rout every enemy unit do win, or lose every friendly unit to lose. Is that not what you are seeing?

Yes, that would be it. Rout them all then. Thanks!