Does it just get annexed back into Italy? The end of the Papacy?
At what time, and for what reason?
The Church is very cautious about taking sides in politics, and with good reason.
Some non-Catholics have the notion that the Pope can dictate to any Catholic. That wasn't true even in the Middle Ages.
Then there is the question of what the Church's interest is.
The mission of the Church is to convey the Grace of God to mankind. That's not a poetic phrase. The Grace of God is the gift of salvation from sin and damnation, won for mankind by Jesus Christ when, incarnated as a man, he suffered and died on the cross. By that act, he atoned for all the sins of mankind. But to receive that grace, one must meet certain conditions: repentance of one's sins, and faith in Jesus as Savior.
The Church imposes other conditions: confession, absolution, and Communion, which may be had only through the Church.
In this matter, the Church is concerned with the eternal salvation or damnation of human souls. Compared to that, any temporal concern interest is nothing. The Church therefore cannot risk its spiritual mission for any temporal reason, however grave.
Thus it would have been wrong for the Church to oppose explicitly a secular power with the potential ability to destroy much of the Church's fabric, including its central administration.
For instance, it has been suggested that the Pope should have excommunicated Hitler or placed an interdict on Germany. Leave aside the probable ineffectuality of such acts. What if the response of the Nazi regime was to set up a "Church of Germany" answering to the German government, like the Church of England, with control of all Church facilities and personnel. Henry VIII was excommunicated too, and all that did was reinforce the separation of England. Or the Nazi regime might strike at the Holy See itself.
Either of these responses could hinder the Church's performance of its spiritual mission, which could mean the damnation of thousands or millions of souls, according to Catholic doctrine. Obviously, this would be a risk that the Pope could not take in good conscience.
So the premise of this thread comes close to ASB land.
Note: I am not a Catholic, nor even religious. But the Pope is, and I am examining the question on the basis of Catholic doctrine.