The outbreak of war between France and Prussia in July 1870 marked the end of the secular power of the Church, as the French occupation troops led by Napoleon III had to move back to their homeland. The Italian government assured the Pope compliance with the agreement of September 1864, allowed, however, when the fortunes of war turned for Napoleon III, to besiege the regions belonging to the church by the army of the Kingdom of Italy.
After the defeat at Sedan and the proclamation of the French Republic, the military siege was intensified, and on 20 September 1870 the royal troops stormed, after a brief gun fire, the "Porta Pia" and marched into Rome. Pius IX wanted to avoid bloodshed and ordered the commander of the Papal forces, General Chancellor, to limit the defence to an absolute minimum to prove that one would only avoid brute force. On the following day the papal troops were dismissed, and only the Swiss Guard remained in the Vatican.
Thus ended a centuries-long period, in which an army under the leadership of the Pope was required to secure the secular power of the church. From then on, the Swiss Guards’ only duty was to protect the life of the Pope and to ensure the safety of the Vatican and the Pope's summer residence in Castel Gandolfo. Therefore, Stalin’s question about how many divisions the Vatican required, made no longer any sense. It shows a too "realistic" and short-sighted view of the facts that determined the course of history.