Plenty of Catholic nations fought wars against the Papal States, for instance, the other Italian states.
That's not completely accurate...
it's true that there were several wars between Italian states, often including the Papal State.
However it was always for territories around the core, never for Rome itself: it would have been inconceivable for a small state to try to conquer the town.
Only big states, aspiring to the imperial title or global dominance did try to conquer Rome.
The Pope had the power to
excommunicate the invader, thus allowing any other power legal right of war against the enemy of the Pope and allowing any pretender to try for a grab for prower.
This is what happened during the
investiture controversy and war between the HRE and Pope.
Some extracts from Wikipedia:
In 1076 Gregory responded by excommunicating the king, removing him from the Church and deposing him as German king. This was the first time a king of his stature had been deposed since the 4th century.
<...snip...>
The German aristocracy was happy to hear of the king's deposition. They used the cover of religion as an excuse to continue the rebellion started at the First Battle of Langensalza in 1075, and for seizure of royal holdings. Aristocrats claimed local lordships over peasants and property, built forts, which had previously been outlawed, and built up localized fiefdoms to secure their autonomy from the empire.
But later on...
In 1081 Henry IV invaded Rome with the intent of forcibly removing Gregory VII and installing a more friendly pope. Gregory VII called on his allies the Normans in southern Italy, and they rescued him from the Germans in 1085. The Normans sacked Rome in the process, and when the citizens of Rome rose up against Gregory he was forced to flee south with the Normans. He died soon thereafter.
And this is only the beginning... on and off there have been a lot of wars between HRE and the papal states with different outcomes.
Lets remember the
Sack of Rome in 1527.
The HRE was not the only one having open war against the Papal States and trying to conquer Rome.
Coming to RFCE I would say:
1. Papal States cannot collapse & independents cannot wage war against it
2. Any playable CIV can declare war against the Papal States but...
2.1 Before the schism, all Christian civs will automatically DoW against the attacker, after the schism, only the Catholics
2.2 Additional -5 relation will be added as well (not permanent) against the attacker
2.3 Before the schism, all towns with christianity will experience unhappiness.
2.4 After the Lutheran reform (printing press) the penalty defined at 2.2 is reduced to from -5 to -3
3 Vassals of the attacker, if Christian (before schism) or Catholic (after schism), will follow their master's decision if their relations are "friendly" else they'll split if relations are less than "friendly" (e.g. cautious).
3.1 The level of relation is evaluated after the -5 relations between them is allocated
3.2 The vassal will have the same penalties as the master (2.1 , 2.2 , 2.3) is will remain together with him after DoW on the Papal States. None of the penalties are allocated is the vassal will split from the master.
3.3 Christian/Catholic vassals of non-Christian/Catholic Civs follow the same rules as other vassals but with a temporary -10 relation penalty
These rules are necessary to allow civs to DoW on the papal states and reproduce the outrage in the Catholic powers about it.
It will possible help a few more religious wars in Europe to match history.
I also would like to change one of the HRE goals to reflect the war between the Emperor and the Pope, with the HRE aspiration of dominate the Italian peninsula.
