In the basic game it'll still happen even if you are allied with them. The act of the Julii becoming at war (no matter who declares it) with Spain will break all Spanish alliances with the other roman factions. As each faction takes it's turn, it will declare war with everyone against whom the Julii are at war. The only times I've seen this work differently are after the roman alliance is broken up by the civil war or one becoming outlawed, or in the base 1.3 patch version which broke the roman diplomacy. It was fixed again in the 1.5 patch. I don't have BI, but I'm pretty sure it's different there.