That happens to me all the time in 1.3. The Roman factions don't stay allied with each other for long. Even though the alliances are broken, I don't think they can attack you. Since my alliance broke with them rather early on, I thought I would attack. I still couldn't because I didn't have enough popularity.

There is an interesting thread over at the offical forums.

http://p223.ezboard.com/fshoguntotal...ID=30994.topic

It appears the Roman factions are now more weary of each other. Also, surprisingly, it is the Julii who start of at war with Carthage rather than the Scipii.