A lot of interesting details to this story. My favorites are:

• in 1870 all of Italy was united except for Rome and the region around it, because that was protected by troops sent by Napoleon III. However, Napoleon III’s France was defeated (through diplomatic and military incompetence) by Bismarck’s Prussia / North German Confederation, which meant the troops were recalled to France. The pope had no chance of defending Rome against the rest of Italy, but he ordered his troops to resist anyways, resulting in several hundred casualties. The pope never surrendered and the Italians didn’t want to force him, so he stayed in his palace until Mussolini of all people signed a treaty with him making the Vatican an independent state.
• the Sack of Rome of 1527 is pretty interesting too.