@bo_sox48, this was a gunboat, no diplomacy at all
@Jamie, mainly the best for italy is to have a french army in marseilles by 1901 autumn. It means he will not build a fleet there. So in many cases italy does nothing with piedmont, so instead of bouncing a french unit (for eg burgundy) he lets a french unit into marseilles, resulting he cant build thre.
Also for France moving to spain and losing Marseilles is neutral, since he gets 1 loses 1, but he is in a better position. That's why I needed the second army as well. Othrwise it's useless to get marseilles (It's useful because you can exchange an italian army to an italian fleet, having 3 fleets and 1 army after marseilles is taken back by france)
So the answer is, france can assume I'm expecting burgundy to move to marseilles, so I don't attack marseilles, because I want his unit in Mars. That's whay he empties it, to be able to build there. It's almost always a 50-50% situation. This time it worked for me, other times I bounce.