A pope is not free to leave the city, to be the head of the Church elsewhere; and in the same way a pope, however well he may understand the modern world, has not the right to relinquish the temporal power.

This is an inalienable inheritance which he must defend, and it is moreover a question of life, peremptory, above discussion.

And thus Leo XIII has retained the title of Master of the temporal dominions of the Church, and this he has done the more readily since as a cardinal--like all the members of the Sacred College when elected--he swore that he would maintain those dominions intact.
Italy may hold Rome as her capital for another century or more, but the coming popes will never cease to protest and claim their kingdom.

If ever an understanding should be arrived at, it must be based on the gift of a strip of territory.

Formerly, when rumours of reconciliation were current, was it not said that the papacy exacted, as a formal condition, the possession of at least the Leonine City with the neutralisation of a road leading to the sea?
Nothing is not enough, one cannot start from nothing to attain to everything, whereas that Civitas Leonina, that bit of a city, would already be a little royal ground, and it would then only be necessary to conquer the rest, first Rome, next Italy, then the neighbouring states, and at last the whole world.