This ist the same situation i had First, you have to beat the spanish, not the Julii! you must hold the Julii on the status quo. If you reconquer Narbonensis, you will have to go into the enemy territory. The spanish want to do peace with you. In my game it always is so. Make the peace contract with Spain and take one, you need only one army, at the border to spain. Spain will never attack if there is a army at their border