It seems to me that factions do not assign you missions directly and out of the blue. It seems that if you make a deal with a faction whereby you do something for them and they then reward you, that deal then becomes a mission. For example:

Italy wants France to attack the Swiss, sends a diplomat to ask them to attack in return for a lot of money.

If the French accept, they get a new mission saying - attack the Swiss. If they complete the mission, the money is then transfered.