Picardy: Picardy isn't very important to French defense. If England or Germany gets a fleet in Picardy, it looks threatening but can't actually do much damage, assuming France doesn't hold Belgium, because France can just keep a fleet in Brest.
However, if someone convoys an army into Picardy, it can be really dangerous because it can push inland, into Paris or Burgundy.
France sometimes moves the Paris army here in Spring 1901; this is called the Picardy Opening. It signals that France is dying to get Belgium and will stop at nothing. This is kind of risky for France, especially if no agreement has been made with England or Germany.
Making an all-out attack on Belgium (Bre -> ENG, Mar -> Bur) is very dangerous because France will only get one build even if Belgium is won, and it's easy for France to end up at the wrong end of the three-way Western triangle.